update version number

Author: pfefferle

README.md

@@ -3,7 +3,7 @@
 **Tags:** OStatus, fediverse, activitypub, activitystream  
 **Requires at least:** 5.5  
 **Tested up to:** 6.6  
-**Stable tag:** 3.0.0  
+**Stable tag:** 3.1.0  
 **Requires PHP:** 7.0  
 **License:** MIT  
 **License URI:** http://opensource.org/licenses/MIT