Update documentation to run the validator locally

We're creating an Open311 app for the City of San Jose and have a number of developers working locally. It'd be great to know how to set this up locally.

[kvlahromei]

Hi @aakashhdesai thanks for your feedback. Did your devs follow the install instructions? Where did the process failed?

You can also use the online instance at validate.open311.org.

@kvlahromei Hey - we haven't had a dev follow the instructions yet, but will update you as we do. We're at the https://codeforsanjose.slack.com/ for #openthreeoneone if you'd like to chat.