Add NEWS entry describing the deprecation of checkout, pull and push

NEWS

@@ -2,6 +2,16 @@
 buildstream 1.3.1
 =================
 
+  o BREAKING CHANGE: The top level commands `checkout`, `push` and `pull` have
+    been moved to the `bst artifact` subcommand group and are now obsolete.
+    For example, you must now use `bst artifact pull hello.bst`.
+
+    The behaviour of `checkout` has changed. The previously mandatory LOCATION
+    argument should now be specified with the `--directory` option. In addition
+    to this, `--tar` is no longer a flag, it is a mutually incompatible option
+    to `--directory`. For example, `bst artifact checkout foo.bst --tar foo.tar.gz`.
+
+
   o Added `bst artifact log` subcommand for viewing build logs.
 
   o BREAKING CHANGE: The bst source-bundle command has been removed. The