Add a link from README.md to CONTRIBUTE.md.

Believe it or not I missed this guide 'till now!
Resolves #73 by addming the ability to customize search boxes
Merge pull request #74 from mbrookes/patch-2

Add a link from README.md to CONTRIBUTE.md.
Remove duplicate Model.model_class
Merge pull request #77 from mbrookes/patch-1

Remove duplicate Model.model_class