Update README.rst

Added instructions to perform arborlabs client setup.
Sure..

<test cmd> is a command that Arborlabs will use for running tests on the repo. Multiple commands could be specified by creating a shell script with the commands in it. 

Btw, if it can be any executable then it means that we don't really need to have it in the repository itself, right? Only if there is a script it needs to be available in that repository.