Add some docs about building new tools

README.txt

@@ -12,3 +12,15 @@ Prerequisites:
 Install:
 
   python setup.py install
+
+Building new tools:
+
+  The tools in this package are meant to be installed with
+  "python setup.py install".  If you want to make a new one,
+  an easier way to develop would be to first install this package,
+  and then copy a specific script like "src/sinefit.py" to a new
+  location, and modify it as desired.
+
+  To add a tool to the package, place it in "src/" and add the
+  appropriate configuration to "setup.py".
+