Completely Redid README

Simplified and pared down the README. (It should be very technical.)
Moved introductory material here:

- <https://github.com/CounterpartyXCP/Community/wiki/Basic-Usage>

- <https://github.com/CounterpartyXCP/Community/wiki/Installation>

- <https://github.com/CounterpartyXCP/Community/wiki/Upgrades>