Add release notes template (#336)

This'll allow us to generate edition release notes automatically,
assuming we're assigning PRs the "protocol" and "protocol-incompat"
labels where appropriate.

Author: magnusbaeck

.github/release.yml

@@ -0,0 +1,12 @@
+---
+changelog:
+  categories:
+    - title: Backwards-incompatible protocol changes
+      labels:
+        - protocol-incompat
+    - title: Protocol changes
+      labels:
+        - protocol
+    - title: Other changes
+      labels:
+        - "*"