update README

Author: paulzfm

README.md

@@ -1,5 +1,57 @@
-# Decaf in Scala
+# The Decaf Compiler, in Scala
 
-Rewriting the Decaf compiler in Scala.
+Decaf is a Java-like, but much smaller programming language mainly for educational purpose.
+We now have at least three different implementations of the compiler in Java, Scala and Rust.
+Since the standard language has quite a limited set of language features, students are welcome to add their own new features.
 
-Implementing...
+## Getting Started
+
+This project requires JDK 12, because the package `decaf.lowlevel` is shared with Java's.
+
+Other dependencies will be automatically downloaded from the maven central repository by the build script.
+
+## Build
+
+Type standard [sbt](https://www.scala-sbt.org) commands after you launch the REPL with `sbt`.
+
+To compile, type:
+
+```
+sbt:decaf-in-scala> compile
+```
+
+To build a standalone jar, type:
+
+```
+sbt:decaf-in-scala> assembly
+```
+
+To start a scala REPL, type:
+
+```
+sbt:decaf-in-scala> console
+```
+
+Or, import the project in IDEA and use sbt plugin, if available.
+
+## Run
+
+In sbt console, type:
+
+```
+sbt:decaf-in-scala> run --help
+```
+
+to show the usage help. Or you first build a standalone jar and run it in CLI.
+
+Possible targets/tasks are:
+
+- PA1: parse source code and output the pretty printed tree, or error messages
+- PA2: type check and output the pretty printed scopes, or error messages
+- PA3: generate TAC (three-address code), dump it to a .tac file, and then output the execution result using our built-in simulator
+- PA3-JVM: generate JVM bytecode
+- PA4: currently same with PA3, will be reserved for students to do a bunch of optimizations on TAC
+- PA5: (default target) allocate registers and emit assembly code, currently we are using a very brute-force algorithm and only generates MIPS assembly code (with pseudo-ops, and no delayed branches)
+
+To run the MIPS assembly code, you may need [spim](http://spimsimulator.sourceforge.net), a MIPS32 simulator.
+For Mac OS users, simply install `spim` with `brew install spim` and run with `spim -file your_file.s`.