Improve README

I might prefer for these instructions to give an explicit example, instead of just describing the general requirements, such as: 

```bash
$ T=$(mktemp --directory)
$ cd $T
$ git clone --recursive 'https://github.com/m-lab-tools/ooni-support.git'
$ cd ooni-support
```

...etc.

However, this suggestion is an enhancement, not a blocker.