[DOC] Update README

Some npm packages require python 2, so the README should list it as a dependency.

Also the README should say something about ripple.txt.
[TASK] Set usable default url in config-example

By changing the domain in config-example to local.ripple.com, the config is usable as a starting point for development by simply copying the file to config.js. It does still require the hosts file to be updated of course.
Merge pull request #1503 from Madsn/update-readme

[DOC] Update README