message: Adds caveat to gdoc
diff: Currently [this Google document](https://docs.google.com/document/d/18nqg36VkDUn594jnHEbXHlZkW_4Bj2x9HIhAgLesSlw/edit#heading=h.nx06tlqgwqzj) serves as the requirements overview.
  +**Note:** We're in process of migrating requirements-tracking to Valispace. Currently the Google is the most consolidated, clean place to track requirements, but Valispace will soon not be the canonical place where requirements are managed.
  +
  ## Requirements documents
  **TODO:** To be actually open source, put exported json in this directory and link nicely from here.
  