newsfragments: Document how to provide release notes

newsfragments/README.md

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+This is the directory for release note fragments processed by
+[towncrier](https://github.com/twisted/towncrier).
+
+When making a user-visible change create a file in this directory and it will be automatically be
+included into the release note document when the next release is published.
+
+The file extension specifies the type of a change. The following are currently supported:
+
+ - .backend: a new feature or a bugfix for a backend.
+ - .frontend: a new feature or a bugfix for a frontend.
+ - .security: a fix for security issue.
+ - .removal: a deprecation or removal of functionality.
+
+Please don't add links to the merge requests into the release notes. If it's not clear where
+the feature is coming from by looking into the git history (e.g. if a release note is being
+made after the MR has landed) then add link to the MR to the commit description.
+
+Please add links to fixed issues in Gitlab by using full URLs to them instead of Gitlab
+shorthand syntax.