Added menhir parser readme

src/haz3lmenhir/README.md

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+# Hazel Menhir Parser
+> This directory contains a Menhir parser for textual Hazel `Exp` syntax and a utility to convert the Menhir AST to post-elaboration `Exp`. This is used for Hazel elaborator unit tests
+* The Menhir parser is located in the `Parser.mly` file
+* The ocamllex lexer can be found in `Lexer.mll`
+* `Interface.re` contains functions to interface with the parser and parse text into an `AST` structure
+* The parser's AST is defined in `AST.re`
+* `Conversion.re` contains utility functions to convert the `AST` structure into the Hazel post-elaboration `Exp` structure
+* Examples of the textual syntax can be found in the `Exp` module of the `TermBase.re` file