[IMP] web_drop_target: Add contributor and screenshot

web_drop_target/readme/CONTRIBUTORS.rst

@@ -1,2 +1,3 @@
 * Holger Brunn <hbrunn@therp.nl>
+* Pablo Fuentes <pablo@studio73.es>
 * Akim Juillerat <akim.juillerat@camptocamp.com>

web_drop_target/readme/USAGE.rst

@@ -1,4 +1,7 @@
 To use this module, you need to:
 
 #. drag a file from your local computer onto an Odoo form view
-#. it should become an attachment of the currently opened record
+#. it should become an attachment of the currently opened record
+
+.. image:: /web_drop_target/static/description/screenshot.png
+    :alt: Screenshot