docs: revise README

[mkovatsc]

Trying to streamline the information given in the README for "new" users (including people who have not used it in a long time).

I noticed that it is complicated with some of the duplication, e.g., https://thingweb.io/docs/node-wot/api/ or https://github.com/eclipse-thingweb/node-wot/blob/master/packages/binding-coap/README.md

We also already saw that some of the references online Things are not reachable, e.g., coap://plugfest.thingweb.io/testthing (neither HTTP nor CoAP).

[mkovatsc]

Note that --save is no longer required since NPM v5.