Change directory structre explanation in accordance to the actual structure.
Add ooni-probe as submodule
Move OONI instructions to docs subdir
Reorganize repo:
  * one directory (excepted docs) for one configuration on the dashboard
  * oonib contains configuration for OONI backend
  * salt contains Salt configuration:
    - salt/relay contains master files for commanding relays
    - salt/probe contains master files for commanding probes
Add README for Salt directory
Add precision to README for Salt