add docs for sa credential generation (#3614) (#2151)

Signed-off-by: Modular Magician <[email protected]>

Author: modular-magician

.changelog/3614.txt

@@ -0,0 +1,3 @@
+```release-note:none
+
+```

website/docs/guides/getting_started.html.markdown

@@ -157,6 +157,10 @@ choose an existing account, or create a new one. Next, download the JSON key
 file. Name it something you can remember, and store it somewhere secure on your
 machine.
 
+> *Note*: Currently the only supported service account credentials are credentials
+downloaded from Cloud Console or generated by `gcloud`.
+
+
 You supply the key to Terraform using the environment variable
 `GOOGLE_APPLICATION_CREDENTIALS`, setting the value to the location of the file.