Add release checklist.

RELEASE.md

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+# Release checklist
+
+1. Run `make check` to check that all tests pass.
+2. Decide on a version number, e.g. 1.0.5
+3. Run `make release VERSION=1.0.5`
+4. Do a `git diff` to check if things look sane.
+4. Do a quick manual test with the `dist` files (via `index.html`)
+5. Run `git push && git push --tags`
+6. Update http://conversejs.org
+7. Create `1.0.5` directory for the CDN.
+8. Run `npm publish`