Add unofficial guide for GitHub SSO

SSO for Github Enterprise was added at 6d63f57, but it seemed not to work with
GitHub.

The curlpits were:

* The url to get the authenticate user is /user, not /api/v3/user.
* Users should set their public email for GitHub.

Describe the detail in GitHub.md and rename the original Github.md to
GitHub-Enterprise.md.

2 files changed, 24 insertions, 0 deletions

diff --git a/doc/integrations/Single-Sign-On/Github.md b/doc/integrations/Single-Sign-On/GitHub-Enterprise.md
index 6f6633846..6f6633846 100644
--- a/doc/integrations/Single-Sign-On/Github.md
+++ b/doc/integrations/Single-Sign-On/GitHub-Enterprise.md
diff --git a/doc/integrations/Single-Sign-On/GitHub.md b/doc/integrations/Single-Sign-On/GitHub.md
new file mode 100644
index 000000000..56e2d1c72
--- /dev/null
+++ b/doc/integrations/Single-Sign-On/GitHub.md
@@ -0,0 +1,24 @@
+## Configuring GitHub Single-Sign-On (unofficial)
+
+Note: Because the authentication interface of GitHub is similar to that of GitLab, the GitLab SSO feature can be used to unofficially also support GitHub SSO.
+
+Follow these steps to configure Mattermost to use Github as a single-sign-on (SSO) service for team creation, account creation and sign-in using the GitLab SSO interface.
+
+1. Login to your GitHub account and go to the Applications section in Profile Settings.
+2. Add a new application called "Mattermost" with the following as Authorization callback URL:
+  * `<your-mattermost-url>` (example: http://localhost:8065)
+
+3. Submit the application and copy the given _Id_ and _Secret_ into the appropriate _GitLabSettings_ fields in config/config.json
+
+4. Also in config/config.json, set _Enable_ to `true` for the _gitlab_ section, leave _Scope_ blank and use the following for the endpoints:
+  * _AuthEndpoint_: `https://github.com/login/oauth/authorize`
+  * _TokenEndpoint_: `https://github.com/login/oauth/access_token`
+  * _UserApiEndpoint_: `https://api.github.com/user`
+
+6. (Optional) If you would like to force all users to sign-up with GitHub only,
+in the _ServiceSettings_ section of config/config.json set _DisableEmailSignUp_
+to `true`.
+
+6. Restart your Mattermost server to see the changes take effect.
+
+7. Tell the users to set their public email for GitHub at the [Public profile page](https://github.com/settings/profile). Mattermost uses the email to create account.