Add TODOs badge to README

[patrickdevivo]

Hi there! I wanted to propose adding the following badge to the README to indicate how many // TODO comments are in this codebase:

The badge links to tickgit.com which is a free service that indexes and displays TODO comments in public github repos. It can help surface latent work and be a way for new contributors to find areas of code to improve.

The markdown is:

[![TODOs](https://img.shields.io/endpoint?url=https://api.tickgit.com/badge?repo=github.com/src-d/go-git)](https://www.tickgit.com/browse?repo=github.com/src-d/go-git)

Thanks for considering, feel free to close this issue if it's not appropriate or you prefer not to!