Update usage examples in README

[bdarcus]

I'm getting has no method errors on the examples in the README. Assuming I'm not doing something wrong (totally possible) could you update them please?

[trentm]

Also README has:

Git = require('git');

shoudl be:

Git = require('git-fs')

... at least after npm install git-fs. :)

[Arro]

I'm confused as well. There's no function .exists or .getTags in the lib/git-fs.js anymore.

[djrenren]

please update this