[PATCH] Document config.mak in INSTALL.

The existing config.mak should satisfy almost everyone...  You
can change the prefix and other vars catch the new setting
anyways.  I had forgotten that ?= acts as = (lazy value binding)
and as not := (immediate value binding).
Signed-off-by: NE. Jason Riedy <ejr@cs.berkeley.edu>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

INSTALL

@@ -87,3 +87,6 @@ Issues of note:
    have all the libraries/tools needed, or you may have
    necessary libraries at unusual locations.  Please look at the
    top of the Makefile to see what can be adjusted for your needs.
+   You can place local settings in config.mak and the Makefile
+   will include them.  Note that config.mak is not distributed;
+   the name is reserved for local settings.