docs(readme): add readme

README.md

@@ -1,4 +1,30 @@
 [![Build Status](https://travis-ci.org/hanks/another-super-tiny-compiler.svg?branch=master)](https://travis-ci.org/hanks/another-super-tiny-compiler) [![Coverage Status](https://coveralls.io/repos/github/hanks/another-super-tiny-compiler/badge.svg)](https://coveralls.io/github/hanks/another-super-tiny-compiler)
 
 # another-super-tiny-compiler
-Python version of the super tiny compiler project
+
+Python version of [the super tiny compiler](https://github.com/thejameskyle/the-super-tiny-compiler) project.
+
+# Why
+
+Just for study, and I am interested in compiler implementation, so I follow this tiny project to
+study the basic ideas to implement a small compiler:)
+
+# Implementation
+
+In this project, the object is to transform `Lisp-style` code to `C-style` code. And there are four phases implemented:
+
+* tokenizer, similar to `lexer` to generate token from source code
+* parser, conduct tokens to AST(abstract syntax tree)
+* transformer, change AST v1 to AST v2
+* code generator, output target code from AST v2
+
+# Demo
+
+```
+> compiler("(add 2 (subtract 4 2))")
+add(2, subtract(4, 2));
+```
+
+# Licence
+
+MIT Licence