doc: fix typos in README

 - Fixed typo: "insallation" -> "installation"
 - Added an "that" in a sentence where it was needed for clarity.

PR-URL: https://github.com/nodejs/node/pull/2852


Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>