Add release step for -sys crates.

Author: jugglerchris

RELEASE.md

@@ -8,6 +8,7 @@
 * Check other fields in Cargo.toml are sensible
 * Check that CI is passing
 * Tag the commit for the release
+* Run `cargo publish` on -sys crates if updated.
 * Run `cargo publish`
 * Check that the version from crates.io looks good
 * Update version number on branch to (next version)-alpha.