Add contribution guidelines and standards section to CONTRIBUTING.md

Several parts are largely based on the post by @yaroslavvb at: tensorflow#7443#issuecomment-279182613

Fixes tensorflow#7443
Change: 154876045

CONTRIBUTING.md

@@ -27,3 +27,140 @@ contributions, often because we probably won't get to them right now. If you
 decide to start on an issue, leave a comment so that other people know that
 you're working on it. If you want to help out, but not alone, use the issue
 comment thread to coordinate.
+
+### Contribution guidelines and standards
+
+Before sending your pull request for
+[review](https://github.com/tensorflow/tensorflow/pulls),
+make sure your changes are consistent with the guidelines and follow the
+TensorFlow coding style.
+
+#### General guidelines and philosophy for contribution
+
+* Include unit tests when you contribute new features, as they help to
+ a) prove that your code works correctly, b) guard against future breaking
+ changes to lower the maintenance cost.
+* Bug fixes also generally require unit tests, because the presense of bugs
+ usually indicates insufficient test coverage.
+* Keep API compatibility in mind when you change code in core TensorFlow,
+ e.g., code in [tensorflow/core](https://github.com/tensorflow/tensorflow/tree/master/tensorflow/core) and [tensorflow/python](https://github.com/tensorflow/tensorflow/tree/master/tensorflow/python).
+ TensorFlow has reached version 1 and hence cannot make
+ non-backward-compatible API changes without a major release. Reviewers of your
+ pull request will comment on any API compatibility issues.
+* When you contribute a new feature to TensorFlow, the maintenance burden is (by
+ default) transferred to the TensorFlow team. This means that benefit of
+ contribution must be compared against the cost of maintaining the feature.
+* Full new features (e.g., a new op implementing a cutting-edge algorithm)
+ typically will live in
+ [tensorflow/contrib](https://github.com/tensorflow/tensorflow/tree/master/tensorflow/contrib)
+ to get some airtime before decision is made regarding whether they are to be
+ migrated to the core.
+
+#### License
+
+Include a license at the top of new files.
+
+* [C/C++ license example](https://github.com/tensorflow/tensorflow/blob/master/tensorflow/core/framework/op.cc#L1)
+* [Python license example](https://github.com/tensorflow/tensorflow/blob/master/tensorflow/python/ops/nn.py#L1)
+* [Java license example](https://github.com/tensorflow/tensorflow/blob/master/tensorflow/java/src/main/java/org/tensorflow/Graph.java#L1)
+* [Go license example](https://github.com/tensorflow/tensorflow/blob/master/tensorflow/go/operation.go#L1)
+* [Bash license example](https://github.com/tensorflow/tensorflow/blob/master/tensorflow/tools/ci_build/ci_sanity.sh#L2)
+* [HTML license example](https://github.com/tensorflow/tensorflow/blob/master/tensorflow/tensorboard/dist/index.html#L2)
+* [JavaScript/TypeScript license example](https://github.com/tensorflow/tensorflow/blob/master/tensorflow/tensorboard/components/tf_backend/backend.ts#L1)
+
+Bazel BUILD files also need to include a license section, e.g.,
+[BUILD example](https://github.com/tensorflow/tensorflow/blob/master/tensorflow/core/BUILD#L61).
+
+#### C++ coding style
+
+Changes to TensorFlow C++ code should conform to
+[Google C++ Style Guide](https://google.github.io/styleguide/cppguide.html).
+
+Use `clang-tidy` to check your C/C++ changes. To install clang-tidy on ubuntu:16.04, do:
+
+```bash
+apt-get install -y clang-tidy
+```
+
+You can check a C/C++ file by doing:
+
+
+```bash
+clang-format <my_cc_file> --style=google > /tmp/my_cc_file.cc
+diff <my_cc_file> /tmp/my_cc_file.cc
+```
+
+#### Python coding style
+
+Changes to TensorFlow Python code should conform to
+[Google Python Style Guide](https://google.github.io/styleguide/pyguide.html)
+
+Use `pylint` to check your Python changes. To install `pylint` and
+retrieve TensorFlow's custom style definition:
+
+```bash
+pip install pylint
+wget -O /tmp/pylintrc https://raw.githubusercontent.com/tensorflow/tensorflow/master/tensorflow/tools/ci_build/pylintrc
+```
+
+To check a file with `pylint`:
+
+```bash
+pylint --rcfile=/tmp/pylintrc myfile.py
+```
+
+#### Coding style for other languages
+
+* [Google Java Style Guide](https://google.github.io/styleguide/javaguide.html)
+* [Google JavaScript Style Guide](https://google.github.io/styleguide/jsguide.html)
+* [Google Shell Style Guide](https://google.github.io/styleguide/shell.xml)
+
+#### Running sanity check
+
+If you have Docker installed on your system, you can perform a sanity check on
+your changes by running the command:
+
+```bash
+tensorflow/tools/ci_build/ci_build.sh CPU tensorflow/tools/ci_build/ci_sanity.sh
+```
+
+This will catch most license, Python coding style and BUILD file issues that
+may exist in your changes.
+
+#### Running unit tests
+
+There are two ways to run TensorFlow unit tests.
+
+1. Using tools and libraries installed directly on your system.
+
+ Refer to the
+ [CPU-only developer Dockerfile](https://github.com/tensorflow/tensorflow/blob/master/tensorflow/tools/docker/Dockerfile.devel) and
+ [GPU developer Dockerfile](https://github.com/tensorflow/tensorflow/blob/master/tensorflow/tools/docker/Dockerfile.devel-gpu)
+ for the required packages. Alternatively, use the said
+ [Docker images](https://hub.docker.com/r/tensorflow/tensorflow/tags/), e.g.,
+ `tensorflow/tensorflow:nightly-devel` and `tensorflow/tensorflow:nightly-devel-gpu`
+ for development to avoid installing the packages directly on your system.
+
+ Once you have the packages installed, you can run a specific unit test in
+ bazel by doing as follows:
+
+ If the tests are to be run on GPU, add CUDA paths to LD_LIBRARY_PATH and add
+ the `cuda` option flag
+
+ ```bash
+ export LD_LIBRARY_PATH="${LD_LIBRARY_PATH}:/usr/local/cuda/lib64:/usr/local/cuda/extras/CUPTI/lib64:$LD_LIBRARY_PATH"
+
+ export flags="--config=opt --config=cuda -k"
+ ```
+
+ For example, to run all tests under tensorflow/python, do:
+
+ ```bash
+ bazel test ${flags} //tensorflow/python/...
+ ```
+
+2. Using Docker and TensorFlow's CI scripts.
+
+ See
+ [TensorFlow Builds](https://github.com/tensorflow/tensorflow/tree/master/tensorflow/tools/ci_build) for details.
+