waf: readme: add an shortcut example using a main product group

The necessity for that was pointed out in the PR #3841 ("waf: readme: update
information regarding groups" -
https://github.com/ArduPilot/ardupilot/pull/3841).

README-WAF.md

@@ -134,6 +134,9 @@ waf bin
 
 # Build all examples
 waf examples
+
+# Build arducopter binaries
+waf copter
 ```
 
 ## Building a specific program ##