Add target "install-html" the the top level Makefile

This makes it possible to install html documents from the top level
directory. Previously such target was only in Documentation/Makefile.

Signed-off-by: Teemu Likonen <tlikonen@iki.fi>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -1290,6 +1290,9 @@ endif
 install-doc:
 	$(MAKE) -C Documentation install
 
+install-html:
+	$(MAKE) -C Documentation install-html
+
 install-info:
 	$(MAKE) -C Documentation install-info