In README.md Sources, document Glean Contrib
* Add Glean::Source::Contrib specs
* Add glean search --contrib switch
* Glean::Source.search searches single source determined by gli switch
* Flesh out Glean::Source::Contrib
  * Define SearchResult to match existing duck type (respond to name and description)
  * Use Glean::Dataset to fetch sources from glean/glean-contrib repo
* Update README examples
Link Glean Contrib to GitHub repo
Merge pull request #8 from simeonwillbanks/glean-contrib

Glean contrib