Clarify contributing guide (#1324)

.github/pull_request_template.md

@@ -0,0 +1,3 @@
+What's this all about?
+
+Fixes: #0000 ⬅️ Be sure to refer to an existing issue here!

docs/contributing.md

@@ -1,8 +1,21 @@
 ## Contributing
 
-This project welcomes contributions and suggestions.  Most contributions require you to agree to a
+Thanks for your interest in C++/WinRT! You are welcome to contribute by filing issues for problems you encounter.
-Contributor License Agreement (CLA) declaring that you have the right to, and actually do, grant us
+Project maintainers will consider changes that improve compatibility or fix bugs. 
-the rights to use your contribution. For details, visit https://cla.opensource.microsoft.com.
+
+The following process is required in order to have a pull request considered:
+
+* File an issue for any change you would like to propose. This can start a discussion so we can agree on an approach
+before you invest a large amount of time. Due to the large number of dependent projects, contributions that include
+compatibility risk or added complexity will generally be rejected. 
+
+* Contributors will need the help of a project maintainer to verify the change with the Windows operating system
+build system. If a project maintainer is available to provide guidance and mentorship for the change, a pull request
+may be opened to begin the formal review process. 
+
+Most contributions require you to agree to a Contributor License Agreement (CLA) declaring that
+you have the right to, and actually do, grant us the rights to use your contribution. For details,
+visit https://cla.opensource.microsoft.com.
 
 When you submit a pull request, a CLA bot will automatically determine whether you need to provide
 a CLA and decorate the PR appropriately (e.g., status check, comment). Simply follow the instructions