add readme

Author: KlotzAndrew

README.md

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+# Regex compiler
+
+Thompson's algorithm for parsing regular expressions
+
+Steps:
+* parse infix to postfix
+* compile postfix to NFA
+* walk nfa with input
+
+Supported grammars:
+  * `|`
+  * `(` `)`
+  * `*`
+
+Inspired by <a href="https://swtch.com/~rsc/regexp/regexp1.html">Regular Expression Matching Can Be Simple And Fast </a> by Russ Cox