Added doc for multiple Android tests

Added documentation for automating multiple Android sessions at once.
I think the wording in this doc isn't consistent with the existing technical documentation.

> `you once again just have to make sure you are `

If you delete the above then it reads:

`If you are using chromedriver or selendroid, set a different port for each server.`

I'm not sure why that line exists though. You always have to set the port regardless.