Add some tips for using make

These are very useful when developing or fixing census scripts.

doc/README

@@ -30,6 +30,19 @@ Makefile: the main makefile machinery, used to setup var dir
 Makefile.var: used to aggregate files and setup derivative dirs
 Makefile.deriv: used by each derivative dir to create files
 
+/-------------------------------------------------
+| Tips for using make
+
+To force re-making a file:
+
+  $ make -B filename
+
+To force re-making a file but force not updating its prerequisites:
+
+  $ make -o prerequisitename -B filename
+
+These are very useful when developing or fixing census scripts.
+
 /-------------------------------------------------
 | Scripts