rb3.rst: Add instructions to prepare and boot from an external MMC sdcard

Change-Id: I11e70e307322855e2598f4ece8827eb1fbc26391
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
   [sumits: updated usage of .. note::]
1 file changed
tree: 5c81fddb0b10977f610a4a307bc6ef92f53f5d44
  1. docs/
  2. .gitignore
  3. .readthedocs.yaml