Add contribution guide [ci skip] (#2034)

Closes #1880

As a recent first-time contributor to puma, there are some of the things
that might have been helpful to have in one place as I worked on my
first changes.

I am also thinking about a resources section that links to the docs
directory and to docs for libraries that might be unfamiliar to someone
looking through puma for the first time (I didn't know much about
TCPSocket, IO.pipe, nio4r, for example)

CONTRIBUTING.md

@@ -0,0 +1,107 @@
+# Contributing to Puma
+
+By participating in this project, you agree to follow the [code of conduct].
+
+[code of conduct]: https://github.com/puma/puma/blob/master/CODE_OF_CONDUCT.md
+
+There are lots of ways to contribute to puma. Some examples include:
+
+* creating a [bug report] or [feature request]
+* verifying bug reports and adding [reproduction steps]
+* reviewing [pull requests] and testing the changes on your own machine
+* writing or editing documentation
+* improving test coverage
+* fixing a bug or adding a new feature
+
+[bug report]: https://github.com/puma/puma/issues/new?template=bug_report.md
+[feature request]: https://github.com/puma/puma/issues/new?template=feature_request.md
+[pull requests]: https://github.com/puma/puma/pulls
+[reproduction steps]: https://github.com/puma/puma/blob/CONTRIBUTING.md#reproduction-steps
+
+## Setup
+
+You will need to install [ragel] to generate puma's extension code.
+
+macOS:
+
+```sh
+brew install ragel
+```
+
+Linux:
+```sh
+apt-get install ragel
+```
+
+Install Ruby dependencies with:
+
+```sh
+bundle install
+```
+
+[ragel]: https://www.colm.net/open-source/ragel/
+
+## Running tests
+
+You can run the full test suite with:
+
+```sh
+bundle exec rake test:all
+```
+
+To run a single test file:
+
+```sh
+ruby -Ilib test/test_integration.rb
+```
+
+Or use [`m`](https://github.com/qrush/m):
+
+```sh
+bundle exec m test/test_binder.rb
+```
+
+Which can also be used to run a single test case:
+
+```sh
+bundle exec m test/test_binder.rb:37
+```
+
+## Reproduction steps
+
+Reproducing a bug helps identify the root cause of that bug so it can be fixed.
+To get started, create a rackup file and config file and then run your test app
+with:
+
+```sh
+bundle exec puma -C <path/to/config.rb> <path/to/rackup.ru>
+```
+
+As an example, using one of the test rack apps:
+[`test/rackup/hello.ru`][rackup], and one of the test config files:
+[`test/config/settings.rb`][config], you would run the test app with:
+
+```sh
+bundle exec puma -C test/config/settings.rb test/rackup/hello.ru
+```
+
+There is also a Dockerfile available for reproducing Linux-specific issues. To use:
+
+```sh
+docker build -f tools/docker/Dockerfile -t puma .
+docker run -p 9292:9292 -it puma
+```
+
+[rackup]: https://github.com/puma/puma/blob/master/test/rackup/hello.ru
+[config]: https://github.com/puma/puma/blob/master/test/config/settings.rb
+
+## Pull requests
+
+Code contributions should generally include test coverage. If you aren't sure how to
+test your changes, please open a pull request and leave a comment asking for
+help.
+
+If you open a pull request with a change that doesn't need to be noted in the
+changelog ([`History.md`](History.md)), add the text `[changelog skip]` to the
+pull request title to skip [the changelog
+check](https://github.com/puma/puma/pull/1991).

README.md

@@ -280,32 +280,9 @@ reliability in production environments:
 
 ## Contributing
 
-To run the test suite:
+Find details for contributing in the [contribution guide].
 
-```bash
-$ bundle install
-$ bundle exec rake
-```
-
-To run a single test file, run only that file:
-
-```bash
-$ ruby -Ilib test/test_integration.rb
-```
-
-Or use [`m`](https://github.com/qrush/m):
-
-```
-$ bundle exec m test/test_binder.rb
-```
-
-Which can also be used to run a single test case:
-
-```
-$ bundle exec m test/test_binder.rb:37
-```
-
-If you open a pull request with a change that doesn't need to be noted in the changelog ([`History.md`](History.md)), add the text `[changelog skip]` to the pull request title to skip [the changelog check](https://github.com/puma/puma/pull/1991).
+[contribution guide]: https://github.com/puma/puma/blob/master/CONTRIBUTING.md
 
 ## License