Adding a Contributing to Stig doc

Issue #154
Typo fix

Closes Issue #154
Merge pull request #155 from vmbrasseur/master

Adding contributing.md
Initial pass on #102

* Add header
* Trim features sections (was very TL;DR)
* Link to build_and_install.md
* Link to contributing.md
Quick system requirement blurb

It doesn't say much, but it says more than it did before which still
makes it one for the Win column.

Issue #102
Add quick start section

Issue #102
Merge pull request #156 from vmbrasseur/master

Update to README.md