infra/rde1edge: add user guide for rd-e1-edge platform

Add the initial user guide for the RD-E1-Edge platform software stack.
The user guide points to documents that provide information about
obtaining the platform software stack and using the various features it
supports. A table to list the release tags is included as well.

Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>
Change-Id: I220490c4d3d1a60219ab012f07b7ac2ca23bc5f2