Add proposals/README.md

Add a new README file for the proposals subdirectory detailing the
contents of the directory and linking to the new proposal process.

proposals/README.md

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+# Summary
+
+This directory contains proposals for web extension APIs, including newly-
+introduced APIs and modifications of existing APIs.
+
+# Adding a new proposal
+
+The process for adding a new proposal is outlined in the
+[proposal process](proposal_process.md), in which you will create a new proposal
+from the [proposal template](proposal_template.md).
+
+# Existing proposals
+
+Proposals here are not "living documents"; that is, they may not be updated
+after an API or API change is implemented. They are not meant to replace a more
+detailed specification, and may be out-of-date.
+
+They are used for discussing upcoming APIs, and committed once there is broad
+consensus on an API.
+
+# Non-normative proposals
+
+A number of proposals were added here before we established the proposal
+process. These do not follow the format outlined in the template. All future
+proposals should follow the documented [process](proposal_process.md) and use
+the [template](proposal_template.md).