Add a CONTRIBUTING file (#198)

Based on the CONTRIBUTING.md in dhall-lang

CONTRIBUTING.md

@@ -0,0 +1,92 @@
+# Contributing to Spago
+
+
+## Do I belong here?
+
+Everybody is welcome! People of all experience levels can join and begin contributing and
+should feel comfortable and safe making mistakes.
+
+People of all backgrounds belong here as long as they treat others with dignity
+and respect and do not harass or belittle others.
+
+
+## What is the correct way to ask a question?
+
+It's ok to ask questions by [opening an issue][spago-issues]!  
+I ([@f-f][f-f]) am also on the [Functional Programming Slack][fp-slack], so you can also ask
+questions in the `#purescript` and `#purescript-beginners` channels.
+
+
+## I'd like to help, how do I pick something to work on?
+
+Any [open issue][spago-issues] that is not assigned to anyone is good to work on!
+
+If it's your first time contributing then it's probably best to pick the ones marked
+with "good first issue".
+
+The easiest way you can help is by contributing documentation (look for issues with
+the label "document me").  
+Do not worry if it's just about copypasting some instructions from an issue to the README,
+everything is welcome!
+
+
+## Developing `spago`
+
+If you'd like to develop spago locally, the recommended tool to use is [stack][stack]
+
+To compile the project from source you can do
+
+```bash
+$ stack build --fast
+```
+
+To install the version you're developing system-wide, do
+
+```bash
+$ stack install
+```
+
+
+## Running tests
+
+The CI runs the tests on new pull requests, so it's not possible to merge a change without them passing.
+
+So you might want to run them locally. This is a way to do it:
+
+```bash
+# Build from source and install system-wide
+$ stack install
+
+# This runs the Python tests which make use of the `spago` executable
+$ cd tests/spago-test
+$ ../spago-test.py
+```
+
+
+## Merging changes
+
+All changes must happen through a Pull Request.
+
+Everyone with the "commit bit" can merge changes.
+
+
+## How do I get the "commit bit"?
+
+Just ask!  I ([@f-f][f-f]) hand out the commit bit freely to anybody who
+displays sustained interest in the project, sometimes even if they don't ask.
+However, you shouldn't assume that you are undeserving if I haven't given you
+the commit bit.  I might just be busy or forgetful, so I appreciate a helpful
+reminder.  You can ask directly by e-mail (see my account profile) or by opening
+an issue, whichever you prefer.
+
+I hand out the commit bit freely because mistakes are easy to fix or roll back.
+Learn by doing and get your hands dirty!
+
+
+[f-f]: https://github.com/f-f
+[stack]: http://haskellstack.org/
+[fp-slack]: https://fpchat-invite.herokuapp.com/
+[spago-issues]: https://github.com/spacchetti/spago/issues
+
+
+

README.md

@@ -518,6 +518,12 @@ $ spago docs
 This will generate all the documentation in the `./generated-docs` folder of your project.
 You might then want to open the `index.html` file in there.
 
+
+## Developing and contributing to Spago
+
+See the [`CONTRIBUTING.md`][contributing]
+
+
 ## FAQ
 
 #### Hey wait we have a perfectly functional `pulp` right?
@@ -676,6 +682,7 @@ So make sure `build` works first. If you still get a failure you might be encoun
 [spago-nix]: https://github.com/justinwoo/easy-purescript-nix/blob/master/spago.nix
 [purescript]: https://github.com/purescript/purescript
 [psc-package]: https://github.com/purescript/psc-package
+[contributing]: CONTRIBUTING.md
 [package-sets]: https://github.com/purescript/package-sets
 [travis-spago]: https://travis-ci.com/spacchetti/spago
 [spago-issues]: https://github.com/spacchetti/spago/issues