readme: show https clone url

The git:// transport is vulnerable to man-in-the-middle and DNS
spoofing attacks and its use over untrusted networks should be
discouraged.

Change to the https:// clone url in the install instructions.
Merge pull request #276 from frasertweedale/fix/readme-clone-url

readme: use https clone url
added documentation for .nvmrc usage
Merge pull request #280 from activars/master

added documentation for .nvmrc usage