Verifying steps to build machinelearning repo on Linux and Mac

[maryamariyan]

Need to verify the instructions in the Documentation folder for building this repo on different distos have no missing steps.

Windows

• Has been verified already.

Unix:

• Linux: clang version 3.9+ is the minimum pre-requisite not 3.5+
• macOS: to be verified.

cc: @danmosemsft @eerhardt

[maryamariyan]

We updated the unix instructions. The docs include link to prerequisites for linux distros and mac os.