Updated Readme.md, added some docs and examples

I've added some links to documentations to help using the gem and making requests to Spreadshirt. 

I've also added an example to create a basket with the spreadshirt API and get the spreadshirt's basket id from the response.
Update README.md

Made some modifications according to https://github.com/mrkamel/spreadshirt_client/pull/2#issuecomment-22324568
Update README.md

Fixed typos.
Updated Readme with feedbacks

Here is the merged code. :)
Update README.md

Fixed some typos
Merge pull request #2 from Kulgar/master

Updated Readme.md, added some docs and examples