add README

Author: sashimi-yzh

README.md

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+Chisel Project Template
+=======================
+
+Another version of the [Chisel template](https://github.com/ucb-bar/chisel-template) supporting mill.
+mill is another Scala/Java build tool without obscure DSL like SBT. It is much faster than SBT.
+
+Contents at a glance:
+
+* `.gitignore` - helps Git ignore junk like generated files, build products, and temporary files.
+* `build.sc` - instructs mill to build the Chisel project
+* `Makefile` - rules to call mill
+* `playground/src/GCD.scala` - GCD source file
+* `playground/src/DecoupledGCD.scala` - another GCD source file
+* `playground/src/Elaborate.scala` - wrapper file to call chisel command with the GCD module
+* `playground/test/src/GCDSpec.scala` - GCD tester
+
+Feel free to rename or delete files under `playground/` or use them as a reference/template.
+
+## Getting Started
+
+First, install mill by referring to the documentation [here](https://com-lihaoyi.github.io/mill).
+
+To run all tests in this design (recommended for test-driven development):
+```bash
+make test
+```
+
+To generate Verilog:
+```bash
+make verilog
+```