Initial draft of CONTRIBUTING.md (#253)

CONTRIBUTING.md

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+# Contributing
+
+## Process
+Most of the standard open-source and Github workflow applies. 
+
+Pull requests are welcome, and maintainers reserve the right to decide what 
+ends up in the codebase. If you want to increase the chances your contribution 
+will make it into the codebase, open an issue describing the features or changes you'd like included before making a PR. 
+Once a PR is opened, maintainers may still request changes.
+
+## Setting up
+
+Make a fork of the repo and clone your fork onto your local machine.
+
+Have a look around! Since `shadow-cljs` is a build tool, there are multiple build configurations in the project's own `shadow-cljs.edn` 
+that you can use to debug and develop features.
+
+## Starting a REPL
+Development is almost entirely REPL-based, so when you're ready to dive in, you'll want to start a REPL.
+
+Starting one and connecting the editor of your choice should be the same as any Leiningen-based project. 
+
+When all else fails, this should work with all nREPL compatible editors:
+
+1. `lein repl`
+2. Establish remote nREPL connection to port in `.nrepl-port`
+
+