chore: update CONTRIBUTING guidelines to talk about more PR criteria (#900)

Author: ianbotsf

CONTRIBUTING.md

@@ -31,16 +31,27 @@ Contributions via pull requests are much appreciated. Before sending us a pull r
 To send us a pull request, please:
 
 1. Fork the repository.
-2. Modify the source; please focus on the specific change you are contributing. If you also reformat all the code, it will be hard for us to focus on your change.
+2. Modify the source; please focus on a specific change in each pull request. If you incorporate multiple unrelated
+   features or reformat all the code, it will be hard for us to focus on your change.
    * Ensure your modifications are accompanied by a [changelog entry](#Changelog) where necessary.
-4. Ensure local tests pass.
-5. Commit to your fork using clear commit messages.
-6. Send us a pull request, answering any default questions in the pull request interface.
-7. Pay attention to any automated CI failures reported in the pull request, and stay involved in the conversation.
+3. Ensure existing local tests pass. Add additional tests to cover the change.
+4. Commit to your fork using clear commit messages.
+5. Send us a pull request, answering any default questions in the pull request interface.
+6. Pay attention to any automated CI failures reported in the pull request, and stay involved in the conversation.
 
 GitHub provides additional document on [forking a repository](https://help.github.com/articles/fork-a-repo/) and
 [creating a pull request](https://help.github.com/articles/creating-a-pull-request/).
 
+### API and style guidelines
+
+The AWS SDK for Kotlin is a multiplatform library intended to work for a wide variety of environments and use cases.
+Please see JetBrains's Kotlin-specific
+[library creators' guidelines](https://kotlinlang.org/docs/jvm-api-guidelines-introduction.html) for general guidance,
+in particular the section on
+[backward compatibility](https://kotlinlang.org/docs/jvm-api-guideliens-backward-compatibility.html). These guidelines
+are not necessarily firm requirements which all contributions must follow, but you should be prepared to discuss PRs
+which deviate from the guidelines.
+
 ### Changelog
 Merges to this repository must include one or more changelog entries which describe the modifications made.