Update documentation about testing after PR review

Thanks to tydeyen for his review of my pull request. I've updated the
information about testing and Solidus development to be more accurate.

guides/getting-started/develop-solidus.md

@@ -50,14 +50,17 @@ rails server
 
 ## Testing
 
-Solidus uses [RSpec](https://rspec.info/) for testing.
+Solidus uses [RSpec](https://rspec.info/) for testing. Refer to its documentation
+for more information about the testing library.
 
 If you intend to submit your work to Solidus as a pull request, it must pass all
-of the Solidus test suites before it is merged. We use CircleCI to run tests on
-all incoming pull requests.
+of the Solidus test suites before it is merged. You must also provide new or
+updated tests for your features or bug fixes.
+
+We use CircleCI to run tests on all incoming pull requests.
 
 To run the test suites for `solidus_frontend` and `solidus_backend`, you need to
-install [PhantomJS](https://phantomjs.org/) on your system first.
+install [ChromeDriver][chomedriver] on your system first.
 
 You can see the build statuses [on our CircleCI status page][circleci].
 
@@ -70,14 +73,14 @@ Solidus project:
 bash build.sh
 ```
 
-This runs using PostgresQL by default, but it can be overridden by specifying
-`DB=sqlite` or `DB=mysql` in the environment. For example:
+This runs using PostgreSQL by default, but it can be overridden by setting the
+`DB` environment variable to `DB=sqlite` or `DB=mysql`. For example: 
 
 ```shell
 DB=mysql bash build.sh
 ```
 
-Note that this will fail if you have not installed PhantomJS on your system.
+Note that this will fail if you have not installed ChromeDriver on your system.
 
 ### Run a single test suite
 
@@ -96,14 +99,6 @@ specify `DB=mysql` or `DB=postgresql` by prepending it to the command:
 DB=postgresql bundle exec rspec
 ```
 
-#### Fail fast
-
-You can enable `FAIL_FAST` to stop any test run after the first failure:
-
-```shell
-FAIL_FAST=true bundle exec rspec spec/models/state_spec.rb
-``` 
-
 ### Generate a code coverage report
 
 You can generate a [SimpleCov](https://github.com/colszowka/simplecov) code
@@ -113,8 +108,6 @@ coverage report by prepending `COVERAGE=true` to the `rspec` command:
 COVERAGE=true bundle exec rspec
 ```
 
-[circleci]: https://circleci.com/gh/solidusio/solidus
-
 ## Develop a Solidus extension
 
 You can add additional features to your store using Solidus extensions. A list
@@ -123,5 +116,7 @@ of supported extensions can be found at [extensions.solidus.io][extensions].
 You can use the [`solidus_cmd`][solidus-cmd] gem if you want to start creating a
 new Solidus extension.
 
+[chromedriver]: https://sites.google.com/a/chromium.org/chromedriver/home
+[circleci]: https://circleci.com/gh/solidusio/solidus
 [extensions]: https://extensions.solidus.io
 [solidus-cmd]: https://github.com/solidusio/solidus_cmd