Add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing
+
+* PR or email
+* this script is POSIX shell
+* use [shellcheck](https://github.com/koalaman/shellcheck) and `checkbashisms.pl` from [debian devscripts](http://http.debian.net/debian/pool/main/d/devscripts/devscripts_2.18.4.tar.xz)
+* whitespaces, no tabs
+
+## Adding a new distro, updating GHC versions, ...
+
+This script makes use of two files:
+
+1. [.download-urls](https://raw.githubusercontent.com/haskell/ghcup/master/.download-urls),
+   which is meta information on what binary tarball to download for the given version, architecture and distribution.
+   If you know your distribution XY works with a tarball, add a `<distroname>=<distrover>` key to that line. `<distroname>`
+   will be the fallback and after that `unknown`. Lines are unique per tarball url.
+2. [.available-versions](https://raw.githubusercontent.com/haskell/ghcup/master/.available-versions),
+   which just lists available upstream versions and tags.
+
+## TODO
+
+- [ ] FreeBSD support ([#4](https://github.com/haskell/ghcup/issues/4))
+- [x] Make fetching tarballs more robust ([#5](https://github.com/haskell/ghcup/issues/5))
+- [x] More code documentation
+- [x] Allow to compile from source ([#2](https://github.com/haskell/ghcup/issues/2))
+- [x] Allow to install cabal-install as well ([#3](https://github.com/haskell/ghcup/issues/3))