feat(CONTRIBUTING): add design proposal guidelines

ripped straight from https://github.com/docker/docker/blob/master/CONTRIBUTING.md#design-and-cleanup-proposals

Author: Matthew Fisher

CONTRIBUTING.md

@@ -44,6 +44,15 @@ This is a rough outline of what a contributor's workflow looks like:
 
 Thanks for your contributions!
 
+### Design Proposals
+
+When considering a design proposal, we are looking for:
+
+- A description of the problem this design proposal solves
+- A pull request that modifies the documentation, describing the feature you are proposing
+- Prefix your pull request with `Proposal:` in the title
+- Please review the existing Proposals before reporting a new one.
+
 ### Commit Style Guideline
 
 We follow a rough convention for commit messages borrowed from CoreOS, who borrowed theirs

docs/contributing/standards.rst

@@ -83,6 +83,17 @@ tests are required for all public methods, although the flake8_ tool used by
 Deis does not enforce this.
 
 
+Design Proposals
+----------------
+
+When considering a design proposal, we are looking for:
+
+- A description of the problem this design proposal solves
+- A pull request that modifies the documentation, describing the feature you are proposing
+- Prefix your pull request with ``Proposal:`` in the title
+- Please review the existing Proposals before reporting a new one.
+
+
 .. _commit_style_guide:
 
 Commit Style