api: Add MOCKED /workflows -endpoint

* Add MOCKED /workflows -endpoint that provides a quick way to
  check if reana-workflow-engine is running. Closes #20.

Signed-off-by: Harri Hirvonsalo <harri.hirvonsalo@cern.ch>

docs/restapi.rst

@@ -5,3 +5,4 @@ POST /yadage
 ------------
 
 .. autofunction:: reana_workflow_controller.app.yadage_endpoint
+.. autofunction:: reana_workflow_controller.app.get_workflows

reana_workflow_controller/app.py

@@ -45,25 +45,37 @@
 @app.route('/workflows', methods=['GET'])
 def get_workflows():
     """Get all workflows.
-    
+
     .. http:get:: /workflows
+
         Returns a JSON list with all the workflows.
-        NOTE: Currently just a mock that can used e.g. to quickly
-        check if reana-workflow-controller is running.
+
+        .. warning::
+            Not implemented, just a mock that can be used e.g. to quickly
+            check if reana-workflow-controller is running.
+
         **Request**:
+
         .. sourcecode:: http
+
             GET /workflows HTTP/1.1
             Content-Type: application/json
             Host: localhost:5000
+
         :reqheader Content-Type: application/json
+
         **Responses**:
+
         .. sourcecode:: http
+
             HTTP/1.1 200 OK
             Content-Length: 22
             Content-Type: application/json
+
             {
               "workflows": {}
             }
+
         :resheader Content-Type: application/json
         :statuscode 200: no error - the list has been returned.
     """