Add badges to repositories

- [ ] License: ![License](https://img.shields.io/github/license/gittools/gittools.core.svg)
- [ ] NuGet: ![NuGet downloads](https://img.shields.io/chocolatey/dt/gitreleasenotes.Portable.svg)
- [ ] CI: [![Build status](https://ci.appveyor.com/api/projects/status/br0rijb3rgn1qb0c/branch/master?svg=true)](https://ci.appveyor.com/project/GitTools/gitreleasenotes/branch/master)