Add Proposal template (#523)

Signed-off-by: Matt Ramotar <mramotar@dropbox.com>

.github/ISSUE_TEMPLATE/proposal.md

@@ -0,0 +1,39 @@
+---
+name: Proposal
+about: Propose an API change
+title: "[Proposal] "
+---
+
+# Proposal: [Title]
+
+Author(s): [GitHub username]
+
+Last updated: [Date]
+
+## Abstract
+
+[A short summary of the proposal.]
+
+## Background
+
+[An introduction of the necessary background and the problem being solved by the proposed change.]
+
+## Proposal
+
+[A precise statement of the proposed change.]
+
+## Rationale
+
+[A discussion of alternate approaches and the trade offs, advantages, and disadvantages of the specified approach.]
+
+## Compatibility
+
+[A discussion of the change with regard to the current version of Store.]
+
+## Implementation
+
+[A description of the steps in the implementation, who will do them, and when.]
+
+## Open issues
+
+[A discussion of open issues relating to this proposal. This section may be omitted if there are none.]