Improving contribution rules
Improving contribution rules

1. adding more details to ease contribution process
2. separating contribution rules into 2 sub-sections
3. removing note about MinkExtension repository creation

Closes #411

minkphp/Mink,jhedstrom/Mink,jordi12100/Mink,jhedstrom/Mink,Behat/Mink,aik099/Mink,minkphp/Mink,Chekote/Mink,talib570/Mink,stof/Mink,handrus/Mink,Chekote/Mink,jordi12100/Mink,talib570/Mink