adds installation instructions to README

[steveliles]

The mismatch between the github repo name and the npm name adds a little friction that's easily solved by an installation section in the README.

Thanks for the lib, its just what i need :)

[jacktuck]

--save is redundant in latest npm i think, @isaacs would know 😆

[steveliles]

Just tried it - you're right, and thanks - I didn't know that had changed!

I looked quickly at the release notes and didn't yet spot when that change arrived ... maybe safer to keep it explicit until the majority are running an npm version for which --save is the default?

Either way is good - the main point for me was to get the npm name on the readme somewhere :)