New recipe: sourcegraph

Hi, I'm a maintainer of the "sourcegraph" package, and I would love for it to be included in melpa.

"sourcegraph" provides the commands:
  - "sourcegraph-search-site" for searching for code on sourcegraph.com
  - "sourcegraph-describe" to look up documentation and type information for an identifier
  - "sourcegraph-describe-show-examples" to get usage examples for an identifier from sourcegraph.com

Url: github.com/sourcegraph/emacs-sourcegraph-mode

Let me know if I missed anything!