Add Cython parser doc

CYTHON_PARSER.md

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+## Information
+The Cython parser is a Cython (C-Python fusion language) module for the StarryPy packer parser. 
+Since it's compiled, it is significantly faster than the pure Python parser. It is distributed as source
+with StarryPy3k and includes a file to compile it quickly. Note that the Cython parser is experimental and
+therefore may contain bugs, does not yet do all of the parsing work, and is disabled by default.
+## Using the Cython parser
+Prerequisites:
+- Cython (built against 0.25.2)
+- A C compiler
+
+Simply run the following command in StarryPy3k's base directory:
+
+`python build_parser.py build_ext --inplace`
+
+Once the Cython parser has been built (as a `.so` on Linux or a `.pyd` on Windows), it will be used automatically.