Update documentation about support for Edge (#334)

Author: marco-c

README.md

@@ -453,13 +453,13 @@ object will contain:
 <td>Edge</td>
 
 <!-- Push without payloads support-->
-<td>✗</td>
+<td>✓ v17+ (April 2018)</td>
 
 <!-- Push with payload support -->
-<td>✗</td>
+<td>✓ v17+ (April 2018)</td>
 
 <!-- VAPID Support -->
-<td>✗</td>
+<td>✓ v17+ (April 2018)</td>
 
 <td></td>
 </tr>