Add simple releasing docs

docs/RELEASING.md

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+# Releasing
+
+Releasing is a simple process -- just create a new GitHub release. Use the
+auto-generate release notes feature, labelling PRs if necessary. The workflows should
+take care of releasing to crates.io and making release assets.
+
+## Versioning
+
+### Major (breaking) changes to languages
+
+These may not be obvious at first, as these are changes made to the data, not the Rust
+code. The following changes to `languages.yaml` are considered breaking:
+
+* Renaming a language
+* Removing a language
+* Removing a matcher (extension, interpreter, glob)[^1]
+
+[^1]: An exception to this is if the matcher is so rarely used that it shouldn't have been added in the first place.