Update README.md

Update README.md to explain how to use the Composer.json file to integrate Mango Pay SDK as a dependency in a PHP project. 

Sample usage of the Mango Pay SDK (installed with Composer) as a Service in a Symfony project.
Tabs where needed

Line 27 and 34
Tabs added for Sample usage for Symfony
Update regarding feedbacks

Hugo, I updated the readme to replace Mango Pay by MangoPay :) 

Thanks for the feedbacks on good practices with Composer. I'm a newbie with all that kind of stuff so I was expecting that someone more experienced, do this job but ... ;) 

Regarding samples,I agree but there was already some in the readme.
Merge pull request #24 from soulghard/patch-1

Update README for Composer installation