added a section that explains rebuilding sqlite3 for node-webkit
Node's website http://nodejs.org/ is hyperlinked instead of the engine's GitHub repository. A local hyperlink to the “INSTALLING” section is introduced instead of less detailed `npm install sqlite3`.
Currently the most simple `npm install sqlite3` is buried in the most deep bottom of the “BUILDING” section. And the instructions for building the `master` branch immediately precede it. This commit creates a separate “INSTALLING” section (placed before “BUILDING”) containing only the most simple instructions. A new method of building the `master` branch is introduced. The user's system won't even need `git` for that because a tarball is downloaded by npm from GitHub. A message about the necessity of meeting node-gyp's requirements is given.
The https://badge.fury.io/ service is used to display the latest version of the project's npm package. A quick look at the GitHub's project page may then tell any user if an upgrade is available (without visiting https://npmjs.org/package/sqlite3 for this purpose). A hyperlink to https://npmjs.org/package/sqlite3 is still available so that the package's version can be confirmed whenever the user feels that https://badge.fury.io/ service is less trusted (or if https://badge.fury.io/ is down, though the latter I haven't experienced yet).