Doc: Updating the user-guides

This documentation patch is updating the software requirements
and build instructions for the 1.0 release

* Removed win and msys2 from supported build enviroments.
* Added a note and guidance for compiling with ARMCLANG 6.10.0
* Added references between build-instructions and requirements
  to improve user experiences

Change-Id: I20c70b9be208eb11b3810bf1e0dc43a4321e1229
Signed-off-by: Minos Galanakis <minos.galanakis@arm.com>