Update README.md for provider (#145)

Author: laurenzlong

src/providers/README.md

@@ -4,3 +4,7 @@ Check out [the Cloud Functions for Firebase documentation](https://firebase.goog
 ### The `datastore` provider
 The Datastore integration is in early preview, and not yet publicly usable.
 If you'd like to request early access to use this integration, [please fill out this form](https://goo.gl/forms/6T2kHu7n4N5IB7Ya2).
+
+Since this is a preview feature, any new version may introduce breaking changes.
+It is recommended that you depend on a specific version of the firebase-functions SDK
+in your project's functions/package.json.