Add contributor guide from pyproject-template (#159)

Fixes #150

CONTRIBUTING.md

@@ -0,0 +1,28 @@
+# Contributing to µfmt
+
+## Setup
+
+Create a fresh development enviroment, and install the
+appropriate tools and dependencies:
+
+ $ make venv
+ $ source .venv/bin/activate
+
+
+## Validate
+
+With the virtualenv activated, run the tests and linters:
+
+ $ make test lint
+
+
+## Submit
+
+Before submitting a pull request, please ensure
+that you have done the following:
+
+* Documented changes or features in README.md
+* Added appropriate license headers to new files
+* Written or modified tests for new functionality
+* Used `make format` to format code appropriately
+* Validated and tested code with `make test lint`