Update contributors list (#948)

- Add @usmanatron

CONTRIBUTORS.md

@@ -348,6 +348,10 @@ The following is a list of contributors to the .NET **nanoFramework** project (a
     <td><img src="https://github.com/Sirokujira.png?size=50" height="50" width="50"></td>
     <td><a href="https://github.com/Sirokujira">Tooru Oonuma</a></td>
   </tr>
+  <tr>
+    <td><img src="https://github.com/usmanatron.png?size=50" height="50" width="50" ></td>
+    <td><a href="https://github.com/usmanatron">Usman Iqbal</a></td>
+  </tr>
   <tr>
     <td><img src="https://github.com/valoni.png?size=50" height="50" width="50"></td>
     <td><a href="https://github.com/valoni">Valon M Hoti</a></td>