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

[![TODOs](https://badgen.net/https/api.tickgit.com/badgen/github.com/pandas-dev/pandas)](https://www.tickgit.com/browse?repo=github.com/pandas-dev/pandas)

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://badgen.net/https/api.tickgit.com/badgen/github.com/pandas-dev/pandas)](https://www.tickgit.com/browse?repo=github.com/pandas-dev/pandas)
```

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