Update README with install as Gem option
Update README with install as Gem option

Closes #30.

slashdotdash/jekyll-lunr-js-search,jphastings/jekyll-lunr-js-search,slashdotdash/jekyll-lunr-js-search,jphastings/jekyll-lunr-js-search,wadaries/jekyll-lunr-js-search,wadaries/jekyll-lunr-js-search