Update the searcher docs to make use of dApp aliases

[bbenligiray]

I updated the dApp workflow so that they never see their dApp ID, only their dApp alias #139

This means that knowing the dApp alias of a dApp and the chain it's operating on, searchers need to compute the respective dApp ID. @api3/contracts exports computeDappId() for that, and the logic of how that works is documented here (in case they don't want to use JS or something).

The searcher docs should be updated accordingly.