Add note documenting weird permissions required (#1727)

* Add note documenting weird permissions required

* Update website/docs/r/repository.html.markdown

Author: kfcampbell

website/docs/r/repository.html.markdown

@@ -10,6 +10,10 @@ description: |-
 This resource allows you to create and manage repositories within your
 GitHub organization or personal account.
 
+~> Note: When used with GitHub App authentication, even GET requests must have
+the `contents:write` permission or else the `allow_merge_commit`, `allow_rebase_merge`,
+and `allow_squash_merge` attributes will be ignored, causing confusing diffs.
+
 ## Example Usage
 
 ```hcl