Add some tips for how to install.

INSTALL

+
+HOW TO INSTALL git-subtree
+==========================
+
+Copy the file 'git-subtree.sh' to /usr/local/bin/git-subtree.
+
+That will make a 'git subtree' (note: space instead of dash) command
+available.  See the file git-subtree.txt for more.
+
+You can also install the man page by doing:
+
+	make doc
+	cp git-subtree.1 /usr/share/man/man1/

Makefile

 default:
 	@echo "git-subtree doesn't need to be built."
+	@echo "Just copy it somewhere on your PATH, like /usr/local/bin."
 	@echo
 	@echo "Try: make doc"
 	@false