Add resources section to README. (#1085)

This PR adds a "Resources" section similar to the RMM README, similar to what I did in cuDF some time ago. My primary reason for adding this is to have a quick link to the RMM documentation.

Authors:
  - Bradley Dice (https://github.com/bdice)

Approvers:
  - Mark Harris (https://github.com/harrism)

URL: #1085