Update README.md, add CONTRIBUTING.md and run folders

Run folders are there so someone doesn't have to manually create them to
follow our instructions, they are excluded in the .gitignore so any
modifications in those should not be recorded by Git.

Signed-off-by: Steven Downer <grinch@outlook.com>
Merge pull request #20 from Grinch/patch-1

Update README.md, add CONTRIBUTING.md and run folders