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
Merge pull request #412 from aik099/contribution-instructions-addon

Improving contribution rules to ensure more streamlined contribution flow
PSR2 standard compliance fixes

1. violations before: 191
2. violations after: 62
3. adding Scrutinizer CI integration

Related to https://github.com/Behat/Mink/issues/414
Merge pull request #419 from aik099/psr2-compliance-fix

PSR2 standard compliance fixes