Added section about using DESTDIR and INSTALL_ROOT.

expat/README

@@ -94,6 +94,19 @@ instructions (after having run "make distclean"):
 
         4. Run "make installlib" (which installs the library only).
            Or, if step 2 was omitted, run "make installlib LIBRARY=libexpatw.la".
+           
+Using DESTDIR or INSTALL_ROOT is enabled, with INSTALL_ROOT being the default
+value for DESTDIR, and the rest of the make file using only DESTDIR.
+It works as follows:
+   $ make install DESTDIR=/path/to/image
+overrides the in-makefile set DESTDIR, while both
+   $ INSTALL_ROOT=/path/to/image make install
+   $ make install INSTALL_ROOT=/path/to/image
+use DESTDIR=$(INSTALL_ROOT), even if DESTDIR eventually is defined in the
+environment, because variable-setting priority is
+1) commandline
+2) in-makefile
+3) environment           
 
 Note for Solaris users:  The "ar" command is usually located in
 "/usr/ccs/bin", which is not in the default PATH.  You will need to