Add a README to src/providers, specifically calling out the non-released status of datastore (#142)

Author: Robert-Jan Huijsman

src/providers/README.md

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+# The Cloud Functions for Firebase Event Providers
+Check out [the Cloud Functions for Firebase documentation](https://firebase.google.com/docs/functions/) for information on how to use the APIs in these packages.
+
+### 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).