Add README for new metadata directory (#2492)

metadata/README.md

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+## Libphonenumber Metadata Tools
+
+This directory contains auxiliary libraries to support reading and manipulating
+the CSV packaged metadata for libphonenumber client library. The initial release
+of these libraries is purely concerned with processing the CSV files, and does
+not yet contain the classes needed to convert the CSV data in the XML and other
+text files used by libphonenumber.
+
+Eventually it is expected that all the tooling for manipulating CSV metadata and
+generating the XML files will be released here, at which time the CSV files will
+become the source of truth for libphonenumber and the XML-based metadata and
+other mapping files will be automatically derived from it.
+
+### Support and Api Stability
+
+**These libraries are not currently supported and do not provide a stable API;
+use at your own risk.**
+
+There are no guarantees of stability for the APIs in this library, and while
+they are not expected to change dramatically, API tweaks and bug fixes are
+inevitable.
+
+### Issues and Contributions
+
+Patches and pull requests cannot be accepted directly on this codebase, so if
+you find an issue with these libraries, please open a new issue for it. However
+we do not accept feature requests, or provide answeres or technical support for
+anything in this directory at this time.
+