Add badges

- https://coveralls.io eg: [![Coverage Status](https://coveralls.io/repos/github/skypjack/entt/badge.svg?branch=master)](https://coveralls.io/github/skypjack/entt?branch=master)
- CircleCI eg: [![CircleCI](https://circleci.com/gh/psykzz/st3-gitblame.svg?style=svg)](https://circleci.com/gh/psykzz/st3-gitblame)