docs(contrib): add contributing guide for submitting pull requests

CONTRIBUTING.md

@@ -5,6 +5,7 @@ You want to contribute? You're awesome! Don't know where to start? Check the [li
 [easy tag]: https://github.com/hyperium/hyper/issues?q=label%3AE-easy+is%3Aopen
 
 
-## Pull Requests
+## [Pull Requests](./docs/PULL_REQUESTS.md)
 
+- [Submitting a Pull Request](./docs/PULL_REQUESTS.md#submitting-a-pull-request)
 - [Commit Guidelines](./docs/COMMITS.md)

docs/PULL_REQUESTS.md

@@ -0,0 +1,49 @@
+# Pull Requests
+
+Pull requests are the way to submit changes to the hyper repository.
+
+## Submitting a Pull Request
+
+In most cases, it a good idea to discuss a potential change in an
+[issue](ISSUES.md). This will allow other contributors to provide guidance and
+feedback _before_ significant code work is done, and can increase the
+likelihood of getting the pull request merged.
+
+### Tests
+
+If the change being proposed alters code (as opposed to only documentation for
+example), it is either adding new functionality to hyper or it is fixing
+existing, broken functionality. In both of these cases, the pull request should
+include one or more tests to ensure that hyper does not regress in the future.
+
+### Commits
+
+Once code, tests, and documentation have been written, a commit needs to be
+made. Following the [commit guidelines](COMMITS.md) will help with the review
+process by making your change easier to understand, and makes it easier for
+hyper to produce a valuable changelog with each release.
+
+However, if your message doesn't perfectly match the guidelines, **do not
+worry!** The person that eventually merges can easily fixup the message at that
+time.
+
+### Opening the Pull Request
+
+From within GitHub, open a new pull request from your personal branch.
+
+Once opened, pull requests are usually reviewed within a few days.
+
+### Discuss and Update
+
+You will probably get feedback or requests for changes to your Pull Request.
+This is a big part of the submission process so don't be discouraged! Some
+contributors may sign off on the Pull Request right away, others may have more
+detailed comments or feedback. This is a necessary part of the process in order
+to evaluate whether the changes are correct and necessary.
+
+Any community member can review a PR and you might get conflicting feedback.
+Keep an eye out for comments from code owners to provide guidance on
+conflicting feedback.
+
+You don't need to close the PR and create a new one to address feedback. You
+may simply push new commits to the existing branch.