NEWS: Add entry for new source-checkout command

NEWS

@@ -2,6 +2,11 @@
 buildstream 1.3.1
 =================
 
+  o A `bst source-checkout` command has been added. This command allows an
+    element's sources to be checkout out into a user specified location. For
+    example, `bst source-checkout foo.bst some/path/` will collect the sources
+    of `foo.bst` and place them in the directory `some/path`.
+
   o BREAKING CHANGE: The 'manual' element lost its default 'MAKEFLAGS' and 'V'
     environment variables. There is already a 'make' element with the same
     variables. Note that this is a breaking change, it will require users to