Add instructions for running tests [ci skip]

This commit adds instructions for running the test suite to help people
start contributing more quickly.

It also mentions standard, introduced in #1404

CONTRIBUTING.md

@@ -12,7 +12,7 @@ Here are some ways *you* can contribute:
 * by suggesting new features
 * by writing or editing documentation
 * by writing specifications
-* by writing code ( **no patch is too small** : fix typos, add comments, clean up inconsistent whitespace )
+* by writing code ( **no patch is too small** : fix typos, add comments, etc. )
 * by refactoring code
 * by closing [issues][]
 * by reviewing patches
@@ -23,13 +23,10 @@ Here are some ways *you* can contribute:
 
 * We use the [GitHub issue tracker][issues] to track bugs and features.
 * Before submitting a bug report or feature request, check to make sure it hasn't
-already been submitted.
+  already been submitted.
-* When submitting a bug report, please include a [Gist][] that includes a stack
+* When submitting a bug report, please include a [reproduction script] and any
-  trace and any details that may be necessary to reproduce the bug, including
+  other details that may be necessary to reproduce the bug, including your gem
-  your gem version, Ruby version, and operating system.  Ideally, a bug report
+  version, Ruby version, and operating system.
-  should include a pull request with failing specs.
-
-[gist]: https://gist.github.com/
 
 ## Cleaning up issues
 
@@ -40,21 +37,63 @@ already been submitted.
   We will happily reopen the issue.
 
 ## Submitting a Pull Request
-1. [Fork][fork] the [official repository][repo].
-2. [Create a topic branch.][branch]
-3. Implement your feature or bug fix.
-4. Add, commit, and push your changes.
-5. [Submit a pull request.][pr]
 
-## Notes
+1. [Fork][fork] the [official repository][repo].
+1. [Create a topic branch.][branch]
+1. Implement your feature or bug fix.
+1. Add, commit, and push your changes.
+1. [Submit a pull request.][pr]
+
+### Notes
+
 * Please add tests if you changed code. Contributions without tests won't be accepted.
 * If you don't know how to add tests, please put in a PR and leave a comment
   asking for help. We love helping!
 * Please don't update the Gem version.
 
+## Running the test suite
+
+The default rake task will run the full test suite and [standard]:
+
+```sh
+bundle exec rake
+```
+
+You can also run a single group of tests (unit, spec, or feature)
+
+```sh
+bundle exec rake spec:unit
+bundle exec rake spec:acceptance
+bundle exec rake features
+```
+
+To run an individual rspec test, you can provide a path and line number:
+
+```sh
+bundle exec rspec spec/path/to/spec.rb:123
+```
+
+You can run tests with a specific version of rails via [appraisal]. To run
+the default rake task against Rails 6, for example:
+
+```sh
+bundle exec appraisal 6.0 rake
+```
+
+## Formatting
+
+Use [standard] to automatically format your code:
+
+```sh
+bundle exec rake standard:fix
+```
+
 [repo]: https://github.com/thoughtbot/factory_bot/tree/master
 [fork]: https://help.github.com/articles/fork-a-repo/
 [branch]: https://help.github.com/articles/creating-and-deleting-branches-within-your-repository/
 [pr]: https://help.github.com/articles/using-pull-requests/
+[standard]: https://github.com/testdouble/standard
+[appraisal]: https://github.com/thoughtbot/appraisal
+[reproduction script]: https://github.com/thoughtbot/factory_bot/blob/master/.github/REPRODUCTION_SCRIPT.rb
 
 Inspired by https://github.com/middleman/middleman-heroku/blob/master/CONTRIBUTING.md