#7 Add Oauth configuration doc with HWIOauth

Currently we only support facebook, twitter and google plus
we use SonataUserBundle User Entity which only provide fields for the three providers above
 #7 move login link for template to documentation and add some useful links
Merge pull request #9 from BenoitLeveque/issue_7-handle-oauth-login

 #7 Add Oauth configuration doc with HWIOauth