README: add policy for new packages

Fixes golang/oauth2#325

Change-Id: I3a9ad8ce1e974bb399356b18819c6b0fe60f7051
Reviewed-on: https://go-review.googlesource.com/c/147459
Reviewed-by: Ross Light <light@google.com>
diff --git a/README.md b/README.md
index 94ffd73..68f436e 100644
--- a/README.md
+++ b/README.md
@@ -70,6 +70,13 @@
 }
 ```
 
+## Policy for new packages
+
+We no longer accept new provider-specific packages in this repo. For
+defining provider endpoints and provider-specific OAuth2 behavior, we
+encourage you to create packages elsewhere. We'll keep the existing
+packages for compatibility.
+
 ## Report Issues / Send Patches
 
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