Adding link to fx-ports in README.md

The command suggested

```sh
$ lsof -i -P | grep -i "b2g"
```
will not work on linux (unless lsof is installed), in that case netstat shall be used.

To avoid all this confusion with finding the port, I wrote a super light library ( [fx-ports](https://github.com/nicola/fx-ports) ) that returns a port and whether it is a firefox or a b2g. At the moment doesn't support Windows, but it will very soon. It is also a command line tool, so one can just `$ fx-ports`.