Update README.md

README.md

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 # python-grako
 Python parser for grako.
+
+### Requirements
+
+This requires grako to be installed. You can do this via pip:
+
+```bash
+pip install grako
+```
+
+###Usage
+
+With grako installed, use this to generate the parser:
+
+```bash
+python -m grako Python.grako -o parser_class.py
+```
+
+And then run the parser on the test code with:
+
+```bash
+python python_parser.py test/test.py start
+```