add rapids-build-backend to docs (#1614)

Fixes #1609
Follow-up to #1502

Adds `rapids-build-backend` to the documentation on how to build `rmm` from source.

Authors:
  - James Lamb (https://github.com/jameslamb)

Approvers:
  - Kyle Edwards (https://github.com/KyleFromNVIDIA)
  - Jiaming Yuan (https://github.com/trivialfis)

URL: #1614

Author: jameslamb

README.md

@@ -69,6 +69,7 @@ GPU Support:
   may work on earlier architectures.
 
 Python requirements:
+* `rapids-build-backend` (available from PyPI or the `rapidsai` conda channel)
 * `scikit-build-core`
 * `cuda-python`
 * `cython`