Update extension-grants.rst with example

docs/misc/extension-grants.rst

@@ -2,7 +2,51 @@
  Extension Grants
 ==================
 
-.. todo:: Describe how to implement extension grants.
+Create a subclass of ``AbstractGrantType`` and create methods `handle` and `saveToken` along with other required methods according to needs
+
+.. code-block:: js
+
+  const OAuth2Server = require('oauth2-server');
+  const AbstractGrantType = OAuth2Server.AbstractGrantType;
+  const InvalidArgumentError = OAuth2Server.InvalidArgumentError;
+  const InvalidRequestError = OAuth2Server.InvalidRequestError;
+
+  class MyCustomGrantType extends AbstractGrantType {
+      constructor(opts) {
+          super(opts);
+      }
+
+      async handle(request, client) {
+          if (!request) throw new InvalidArgumentError('Missing `request`');
+          if (!client) throw new InvalidArgumentError('Missing `client`');
+
+          let scope = this.getScope(request);
+          let user = await this.getUserBySomething(request);
+
+          return this.saveToken(user, client, scope);
+      }
+
+      async saveToken(user, client, scope) {
+          this.validateScope(user, client, scope);
+
+          let token = {
+              accessToken: await this.generateAccessToken(client, user, scope),
+              accessTokenExpiresAt: this.getAccessTokenExpiresAt(),
+              refreshToken: await this.generateRefreshToken(client, user, scope),
+              refreshTokenExpiresAt: this.getRefreshTokenExpiresAt(),
+              scope: scope
+          };
+
+          return this.model.saveToken(token, client, user);
+      }
+
+      async getUserBySomething(request) {
+          //Get user's data by corresponding data (FB User ID, Google, etc.), etc.
+      }
+  }
+
+  module.exports = MyCustomGrantType;
 
 Extension grants are registered through :ref:`OAuth2Server#token() <OAuth2Server#token>` (``options.extendedGrantTypes``).
 
+This might require you to approve the new ``grant_type`` for a particular ``client`` if you do checks on valid grant types.