Putting selectors.md back

[dobooth]

Description

This file was removed to mitigate build errors. We can put it back now.

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/verification tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)
• Changes to Framework doesn't have backward incompatible changes for tests or have related Pull Request with fixes to tests

[coveralls]

Coverage increased (+0.6%) to 55.107% when pulling f5e0799 on magento-devdocs:db_selectors into 8e2fa00 on magento:master.