Add an auth provider guide for Mattermost

Signed-off-by: Ondřej Slabý <kron258@gmail.com>

README.md

@@ -202,7 +202,7 @@ There are some config settings you need to change in the files below.
 | `CMD_GITLAB_CLIENTID` | no example | GitLab API client id |
 | `CMD_GITLAB_CLIENTSECRET` | no example | GitLab API client secret |
 | `CMD_GITLAB_VERSION` | no example | GitLab API version (v3 or v4) |
-| `CMD_MATTERMOST_BASEURL` | no example | Mattermost authentication endpoint |
+| `CMD_MATTERMOST_BASEURL` | no example | Mattermost authentication endpoint for versions below 5.0. For Mattermost version 5.0 and above, see [guide](docs/guides/auth/mattermost-self-hosted.md). |
 | `CMD_MATTERMOST_CLIENTID` | no example | Mattermost API client id |
 | `CMD_MATTERMOST_CLIENTSECRET` | no example | Mattermost API client secret |
 | `CMD_DROPBOX_CLIENTID` | no example | Dropbox API client id |

docs/guides/auth/mattermost-self-hosted.md

@@ -0,0 +1,58 @@
+Authentication guide - Mattermost (self-hosted)
+===
+
+*Note: The Mattermost setup portion of this document is just a quick guide. See the [official documentation](https://docs.mattermost.com/developer/oauth-2-0-applications.html) for more details.*
+
+This guide uses the generic OAuth2 module for compatibility with Mattermost version 5.0 and above.
+
+1. Sign-in with an administrator account to your Mattermost instance
+2. Make sure **OAuth 2.0 Service Provider** is enabled in the Main Menu (menu button next to your username in the top left corner) --> System Console --> Custom Integrations menu, which you can find at `https://your.mattermost.domain/admin_console/integrations/custom`
+
+![mattermost-enable-oauth2](../images/auth/mattermost-enable-oauth2.png)
+
+3. Navigate to the OAuth integration settings through Main Menu --> Integrations --> OAuth 2.0 Applications, at `https://your.mattermost.domain/yourteam/integrations/oauth2-apps`
+4. Click on the **Add OAuth 2.0 Application** button to add a new OAuth application
+
+![mattermost-oauth-app-add](../images/auth/mattermost-oauth-app-add.png)
+
+5. Fill out the form and click **Save**
+
+![mattermost-oauth-app-form](../images/auth/mattermost-oauth-app-form.png)
+
+*Note: The callback URL is \<your-codimd-url\>/auth/oauth2/callback*
+
+6. After saving the application, you'll receive the Client ID and Client Secret
+
+![mattermost-oauth-app-done](../images/auth/mattermost-oauth-app-done.png)
+
+7. Add the Client ID and Client Secret to your config.json file or pass them as environment variables
+    * config.json:
+      ````javascript
+      {
+        "production": {
+          "oauth2": {
+              "baseURL": "https://your.mattermost.domain",
+              "userProfileURL": "https://your.mattermost.domain/api/v4/users/me",
+              "userProfileUsernameAttr": "id",
+              "userProfileDisplayNameAttr": "username",
+              "userProfileEmailAttr": "email",
+              "tokenURL": "https://your.mattermost.domain/oauth/access_token",
+              "authorizationURL": "https://your.mattermost.domain/oauth/authorize",
+              "clientID": "ii4p1u3jz7dXXXXXXXXXXXXXXX",
+              "clientSecret": "mqzzx6fydbXXXXXXXXXXXXXXXX"
+          }
+        }
+      }
+      ````
+    * environment variables:
+      ````
+      CMD_OAUTH2_BASEURL=https://your.mattermost.domain
+      CMD_OAUTH2_USER_PROFILE_URL=https://your.mattermost.domain/api/v4/users/me
+      CMD_OAUTH2_USER_PROFILE_USERNAME_ATTR=id
+      CMD_OAUTH2_USER_PROFILE_DISPLAY_NAME_ATTR=username
+      CMD_OAUTH2_USER_PROFILE_EMAIL_ATTR=email
+      CMD_OAUTH2_TOKEN_URL=https://your.mattermost.domain/oauth/access_token
+      CMD_OAUTH2_AUTHORIZATION_URL=https://your.mattermost.domain/oauth/authorize
+      CMD_OAUTH2_CLIENT_ID=ii4p1u3jz7dXXXXXXXXXXXXXXX
+      CMD_OAUTH2_CLIENT_SECRET=mqzzx6fydbXXXXXXXXXXXXXXXX
+      ````