Add instructions for running Black on save in Vim

README.md

@@ -528,6 +528,12 @@ install *Black* (for example you want to run a version from master),
create a virtualenv manually and point `g:black_virtualenv` to it.
The plugin will use it.

To run *Black* on save, add the following line to `.vimrc` or `init.vim`:

```
autocmd BufWritePost *.py execute ':Black'
```

**How to get Vim with Python 3.6?**
On Ubuntu 17.10 Vim comes with Python 3.6 by default.
On macOS with Homebrew run: `brew install vim --with-python3`.