Update docs with gem installation info.
Update docs with gem installation info.

anabhilash619/github,davengeo/github,wicky-info/github,yaoxiaoyong/github,joshsoftware/github,befairyliu/github,GistBox/github,cocktail-io/github,piotrmurach/github,durai145/github,peter-murach/github,firstval/github,MalachiTatum/github,samphilipd/github