Add tip in the README to avoid breaking changes

[dbeatty10]

As mentioned in #42 (review), here's something we should probably add to the README:

Tip

We recommend clone and install as a local package, or just copy-paste.
But if you choose to ignore that recommendation and use subdirectory within packages.yml to install directly from this repo instead:

To avoid breaking behavior changes affecting your dbt project, you'll probably want to
specify a 40-character hash as the revision in addition to configuring the subdirectory. That way, you won't accidentally receive breaking changes.