Add GitHub template for meeting proposals

We now track the schedule and proposals for the WG-async Thursday
meetings on a GitHub project board:

https://github.com/orgs/rust-lang/projects/40

This means that we file issues for each meeting and meeting proposal.
Let's make this easier to do correctly by adding a GitHub issue
template for creating these issues.

.github/ISSUE_TEMPLATE/meeting-proposal.md

@@ -0,0 +1,21 @@
+---
+name: Meeting proposal
+about: Propose a topic for a meeting.
+title: "(My meeting proposal)"
+labels: meeting-proposal, WG-async
+assignees: ""
+---
+
+# Summary
+
+*Describe your meeting topic here.  It doesn't have to be long, but it's always good to try and identify a concrete question to be answered or narrow topic for discussion.*
+
+# Background reading
+
+*Include any links to material that people might read before-hand.*
+
+# About this issue
+
+This issue is a proposal for a WG-async [meeting][].
+
+[meeting]: https://rust-lang.github.io/wg-async/meetings.html