Clarify readme

I couldn't figure out from the readme how this is different from setting custom search keywords in Chrome: I've gone into the search engines list and set hoogle.haskell.org to !h so I can type `!h query` into my omnibox and search hoogle with it. This seems to implement the same or a similar thing? Or am I misunderstanding?

Start with the README and review the issue's comparison with Chrome custom search keywords and Hoogle. Clarify whether Rusty Skywalker's bookmark search differs from that workflow, and consider the README complete when a new reader can understand the distinction without asking this question.

Written by the indexing model from the issue text.