clarified using PREFIX on Installing

docs/manual/installing.tex

@@ -83,6 +83,9 @@ \section{Building And Installing}
 There is no \shell{./configure} for Mongrel2 since we avoid too many OS specific
 differences or shield those away with good feature checks in the code.
 
+If you want to install to a different location than the default  ``/usr/local``,
+use \shell{PREFIX=/path/to/somewhere make all install} instead.
+
 The end result of this should be:
 
 \begin{enumerate}