Add NEWS for guessing the element in commands

This is a part of #222

NEWS

@@ -82,6 +82,11 @@ buildstream 1.3.1
   o Opening a workspace now creates a .bstproject.yaml file that allows buildstream
     commands to be run from a workspace that is not inside a project.
 
+  o Specifying an element is now optional for some commands when buildstream is run
+    from inside a workspace - the 'build', 'checkout', 'fetch', 'pull', 'push',
+    'shell', 'show', 'source-checkout', 'track', 'workspace close' and 'workspace reset'
+    commands are affected.
+
 
 =================
 buildstream 1.1.5