improve documentation for theme-gem installation (#6387)
improve documentation for theme-gem installation (#6387)

Merge pull request 6387
jekyll/jekyll,jekyll/jekyll,chrisfinazzo/jekyll,Strangehill/jekyll,getandpost/jekyll.github.io,chrisfinazzo/jekyll,ryanshaw/jekyll,Strangehill/jekyll,ryanshaw/jekyll,tillgrallert/jekyll,jaybe-jekyll/jekyll,ryanshaw/jekyll,getandpost/jekyll.github.io,Strangehill/jekyll,jaybe-jekyll/jekyll,x-way/jekyll,fulldecent/jekyll,x-way/jekyll,chrisfinazzo/jekyll,tillgrallert/jekyll,gencer/jekyll,x-way/jekyll,tillgrallert/jekyll,fulldecent/jekyll,x-way/jekyll,jekyll/jekyll,Strangehill/jekyll,fulldecent/jekyll,gencer/jekyll,Strangehill/jekyll,fulldecent/jekyll,tillgrallert/jekyll,gencer/jekyll,tillgrallert/jekyll,gencer/jekyll,jekyll/jekyll,chrisfinazzo/jekyll,jaybe-jekyll/jekyll,ryanshaw/jekyll,getandpost/jekyll.github.io,jaybe-jekyll/jekyll,yhironaka/yhironaka.github.io,gencer/jekyll