Create proposals README.md

Author: aterrel

docs/proposals/README.md

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+# DISCOVER Cookbook Proposals
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+DISCOVER Cookbook Proposals, inspired by Matplotlib Enhancement Proposals (MEP), are design documents for large changes.  These documents should provide a discussion of both why and how the changes should be made.
+
+To create a new proposal open a pull request (PR) adding a file based on the template in this direcotry.  For the initial PR only a rough description is required and it should be merged quickly.  Further detailed discussion can happen in follow on PRs or on the [discussion board](https://github.com/numfocus/DISCOVER-Cookbook/discussions).