Update tools.md

Added Trello, formatting.

Author: msmichellegar

tools.md

@@ -1,15 +1,17 @@
-Tools used to create presentation
-
-Useful sites
+####**Useful tools for wireframing and user stories:**
 
 - Google slides
-    https://docs.google.com/presentation/
+    https://docs.google.com/presentation/  
     Used to create the wireframe designs and mock ups
 
 - Canva
-    http://www.canva.com/
+    http://www.canva.com/  
     Used to create simple graphics and fonts (online photoshop equivalent)
 
 - Pivotal Tracker
-    http://www.pivotaltracker.com/
+    http://www.pivotaltracker.com/  
     Used to create and manage digital user stories/agile software development
+
+- Trello
+    https://trello.com/  
+    Another alternative for creating digital versions of user story cards