Add Contributing section to README.md
Add Contributing section to README.md

Both to encourage contributions and to give some advice on how to go
about it.

Signed-off-by: Kevin Locke <ffb4761cba839470133bee36aeb139f58d7dbaa9@kevinlocke.name>

kevinoid/travis-status,kevinoid/travis-status,kevinoid/node-project-template,kevinoid/promised-read,kevinoid/git-branch-is,kevinoid/swagger-spec-validator,kevinoid/git-branch-is,kevinoid/appveyor-status,kevinoid/travis-status,kevinoid/git-branch-is,kevinoid/stream-compare,kevinoid/swagger-spec-validator,kevinoid/promised-read,kevinoid/stream-compare,kevinoid/appveyor-status,kevinoid/node-project-template,kevinoid/promised-read,kevinoid/appveyor-status,kevinoid/node-project-template,kevinoid/stream-compare,kevinoid/swagger-spec-validator