Merge pull request #534 from thomasdarde/patch-2

Update Migrating guide

MIGRATING.md

@@ -74,6 +74,8 @@ are expected to be added by end of Q2 2015.
 The underlying [Signet](https://github.com/google/signet) is still used for authorization. OAuth 2 credentials obtained
 previously will continue to work with the `0.9` version. OAuth 1 is no longer supported.
 
+If you were using a PKCS12 file to authorize, we recommend you generate a new key for the service account using the JSON format ( client_secret.json) file with googleauth.
+
 ## Media uploads
 
 Media uploads are significantly simpler in `0.9`.