Clarify requirements for setting up pa11y-dashboard

[josebolos]

We're having this recurrent problem where people get confused by the install instructions, thinking that pa11y-webservice requires some manual configuration as specified in the pa11y-webservice setup guide.

We should make it clear in both pa11y-dashboard's and pa11y-webservice's READMEs that manually configuring pa11y-webservice is only required if you're going to use it as a standalone app, and if you're using pa11y-dashboard all the configuration needs to be done exclusively through the dashboard's config.