renaming mom builtin to macro

lib/mom/macro/README.md

@@ -0,0 +1,19 @@
+## Builtin module
+
+The Builtin module contains functions that can not be coded in ruby.
+It is the other side of the parfait coin, part of  the runtime.
+
+The functions are organised by their respective classes and get loaded in boot_classes! ,
+right at the start. (see register/boot.rb)
+
+These functions return their code, ie a Parfait::CallableMethod with a MethodSource object,
+which can then be called by ruby code as if it were a "normal"  function.
+
+A normal ruby function is one that is parsed and transformed to code. But not all
+functionality can be written in ruby, one of those chicken and egg things.
+C uses Assembler in this situation, we use Builtin functions.
+
+Slightly more here : http://ruby-x.org/2014/06/10/more-clarity.html (then still called Kernel)
+
+The Builtin module is scattered into several files, but that is just so the file
+doesn't get too long.