Move express parser into module code

src/ifcopenshell-python/ifcopenshell/express/README.txt

@@ -0,0 +1,12 @@
+This folder contains Python code to generate C++ type information based on an
+Express schema. In particular is has only been tested using recent version of
+the IFC schema and will most likely fail on any other Express schema.
+
+The code can be invoked in the following way and results in several code outputs
+named according to the schema name in the Express file. A python 3 interpreter
+with the pyparsing [1] library is required.
+
+$ python bootstrap.py express.bnf > express_parser.py
+$ python express_parser.py IFC2X3_TC1.exp header implementation schema_class definitions
+
+[1] http://pyparsing.wikispaces.com/Download+and+Installation