Makefile: Mention usual targets of subdir Makefiles

Suggested-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
CC: M A Young <m.a.young@durham.ac.uk>
CC: Andrew Cooper <andrew.cooper3@citrix.com>

Makefile

@@ -90,6 +90,11 @@ test:
 #
 # Also, we have a number of targets COMPONENT which run
 # dist-COMPONENT, for convenience.
+#
+# The Makefiles invoked with -C from the toplevel should
+# generally have the following targets:
+#       all  build  install  clean  distclean
+
 
 .PHONY: dist
 dist: DESTDIR=$(DISTDIR)/install