Add README

Author: ksss

gems/actionpack/README.md

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+Maintenance policy for RBS of Rails
+===
+
+We basically only accept changes for the versions listed under the **"List of currently supported releases"** in the [Rails' maintenance policy](https://rubyonrails.org/maintenance).
+
+Versions outside of that are generally not accepted for changes. We do not run tests for them, nor do we include them in format detection.
+
+This is a measure to reduce maintenance costs.
+
+If you have any questions, please contact the maintainers. Refer to `_reviewers.yaml` for the list of maintainers.
+
+## Target gems
+
+- actioncable
+- actionmailer
+- actionpack
+- actiontext
+- actionview
+- activejob
+- activemodel
+- activerecord
+- activestorage
+- activesupport
+- railties
+
+## See also
+
+https://github.com/ruby/gem_rbs_collection/issues/788