copied GNU/Linux distro installation instructions from https://github.com/agraef/purr-data/wiki/Installation, and removed references between instructions for different platforms

update contributor guide, making clear a prototyping language cannot itself be build out of prototypes

Some gentle reformatting to make the Markdown more standards-compliant, so that it renders correctly on Github.

* Macports build instructions were incomplete. The main blocker was that
packages/darwin_app/embed-MacOSX-dependencies.sh is hard-coded to use
homebrew paths instead of Macports or Fink paths. That could be fixed,
but there are bigger problems:
a) having two build environments splits the developer knowledge-base in
half and potentially doubles the number of build issues
b) Macports requires full XCode which is multi-gigs of download and
probably hours more of installation time. In fact I ran out of space on
my OSX VM just from the App Store trying to update XCode!