docs: Create initial Versions documentation (#183)

Motivated by related discussions during review of #168.

Signed-off-by: Michael Vorburger <mike@vorburger.ch>

docs/versions.md

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+# Versions & Backwards Compatibility
+
+This project "versions" external nullness annotations for public & protected API surfaces of common Java libraries.
+
+In some cases we maintain both older & newer versions; e.g. for the JDK, or the Servlet API in `javax` and `jakarta` packages, etc.
+
+In other cases in our next major version release we may remove old versions of libararies which we believe to longer be used much.
+
+Of course, you can always use already stable past versions released to Maven central, even if we've cleaned them out at source.
+
+Contributions for additional versions which you need in your projects are always welcome.