README.rst: update npm package installation instruction

I played with original instruction that installs nib in local directory, the
result is that npm is not able to find nib package.

Install nib package in global mode to avoid this.

Signed-off-by: Wei Liu <liuw@liuw.name>
Merge pull request #80 from liuw/readme-npm-package-install

README.rst: update npm package installation instruction