feat(backend-github): Open Authoring (#2430)

* Make filterPromises resolve entries before filtering

* Add filterPromisesWith & onlySuccessfulPromises to utility library

* Memoize user method in GitHub API

* Make storeMetadata safe to call concurrently in GitHub API

* Fork workflow: startup and authentication

* Fork workflow: backend support

* Fork workflow: disable unused UI elements

* Fork workflow: docs

* Fork workflow: fix deploy previews

* Suggested edits for fork workflow doc

* Change future tense to present

* Fork workflow: add beta status to docs

* remove debug statement

* rename fork workflow to Open Authoring

website/content/docs/beta-features.md

@@ -227,3 +227,11 @@ Template tags produce the following output:
 - `{{collection}}`: the name of the collection containing the entry changed
 
 - `{{path}}`: the full path to the file changed
+
+### Open Authoring
+
+When using the [GitHub backend](/docs/authentication-backends/#github-backend), you can use Netlify CMS to accept contributions from GitHub users without giving them access to your repository. When they make changes in the CMS, the CMS forks your repository for them behind the scenes, and all the changes are made to the fork. When the contributor is ready to submit their changes, they can set their draft as ready for review in the CMS. This triggers a pull request to your repository, which you can merge using the GitHub UI.
+
+At the same time, any contributors who _do_ have write access to the repository can continue to use Netlify CMS normally.
+
+More details and setup instructions can be found on [the Open Authoring docs page](/docs/open-authoring).