docs: Add writeup for test instructions etc

tests/README.md

@@ -9,3 +9,49 @@ Files:
 1. `conftest.py`: Location of pytest global and configuration fixtures.
 2. `utils.py`: Location of global Python helper objects (i.e. constants and `Ozone` instance) to use in tests.
 3. `test_*.py`: Test files, each file is testing one public method.
+
+# Setting up and running tests
+
+After setting development environment as pointed out in CONTRIBUTING.md, you should already have all necessary testing packages installed. To run all tests, invoke this command from the root directory.
+
+```sh
+pytest
+```
+
+# Updating tests
+
+Generally, tests should correspond to the necessary specification/expectation of Ozone users. Tests will help us identify if our code is still in line with such expectations.
+
+Tests should be updated when e.g.:
+- There is a new functionality. In this case, add necessary tests accordingly.
+- There is a change in existing functionality that is in line with the expectation of Ozone users. In this case, the tests become outdated and need to be updated.
+- There is a new bug or previously unencountered or undocumented behavior. Add them to the existing tests to make sure the same bug will never slip past again in the future.
+
+# About the cassettes and pytest-recording
+
+This test suite is testing Ozone's functionality, so interaction with outside sources need to be mocked. Ozone uses pytest-recording plugin that uses vcrpy under the hood to record request-response pairs. These request-response pairs are stored as `.yaml` files in `tests/cassettes` directory.
+
+The `test/cassettes` directory is organized as follows:
+
+- Each folder per one test file.
+- Each `.yaml` file per one test function.
+
+For the purposes of this test suite, these request-response pairs are **assumed** to be all correct. In the event that WAQI API changes their specifications (unlikely), these request-response pairs need to be re-recorded. See the next section.
+
+# Adding or updating cassettes
+
+By default, pytest-recording will only use existing cassettes for testing, and will raise error if there's a new interaction that is not contained in existing cassettes. This prevents "accidentally" making a request that is not already been mocked.
+
+This default behavior can be overridden by giving a `record-mode` argument when invoking `pytest`:
+
+- `pytest --record-mode=once` to write new cassettes but not new request-response pairs within existing cassettes.
+- `pytest --record-mode=new_episodes` to write new interactions, even when there's an existing cassette.
+- `pytest --record-mode=rewrite` to rewrite all existing cassettes.
+- `pytest --record-mode=all` to record all interactions again.
+- `pytest --record-mode=none`(default) to only use existing cassettes and raise error if there's a new request that is not recorded in the existing cassettes.
+
+> To have more confidence that the tests will not go over the wire, the `--block-network` flag can also be passed to block all network access.
+
+For more information about pytest-recording:
+- [pytest-recording homepage](https://github.com/kiwicom/pytest-recording)
+- [VCRpy documentation about record modes](https://vcrpy.readthedocs.io/en/latest/usage.html#record-modes)