dev: Document the release process (first draft).

release.md

@@ -0,0 +1,11 @@
+# Release procedure (for developers):
+
+- Check open issues and outstanding pull requests
+- Confirm `make test` passes
+  - check with multiple zsh versions
+- Update changelog.md
+- Update ./.version
+- `git tag -am "Release $(<.version)" zsh-syntax-highlighting-"$(<.version)" && git push --tags`
+- `perl -pi -e 's/$/-dev/' ./.version`
+- Notify downstreams (OS packages)
+  - anitya should autodetect the tag