added small guide for auth

docs/guides/auth.md

@@ -0,0 +1,15 @@
+# Guide - Authentication
+
+### Google
+* Be sure to enable Google Plus API in your Google account before using Google authentication in this application
+
+### GitLab
+* Previous versions required the user to upload a GitLab profile image for GitLab authentication to work correctly, you may need to pull the newest version to skip this issue
+* For `baseURL`, use the `https` protocol
+* There has been known issues with custom certificates, check that you are running Node v7+ with Hackmd
+* For `scope`, GitLab currently asks users to specify `read_user` and / or `api` options, Hackmd defaults to the `api` option, you'll need to specify `read_user` for user authentication ONLY
+
+### OAuth
+* If you're using OAuth for third-party integrations (Facebook, Twitter, Github, etc.), you'll need to specify callback URLs according to the patterns [here](https://github.com/hackmdio/hackmd#third-party-integration-oauth-callback-urls)
+* Example: `http://hackmd.example.com/auth/gitlab/callback`
+