[Docs] Added Information about Secure Monitor

This commit makes the following changes:
- Adds information about secure monitor.
- Explains that most OSs are supported.
  Points reader to README for build instructions.
- Tags Android-centric information as such.

Bug 1510390
Bug 1627133

Change-Id: I356610c72ed64e0d3747105c08330fdfa861867e
Reviewed-on: http://git-master/r/823053
Tested-by: Kathy Stone <kstone@nvidia.com>
Reviewed-by: Varun Wadekar <vwadekar@nvidia.com>
1 file changed