Doc: Add the ReST usage guidelines.

The guidelines contain how to tag content using the ReST markup employed
by Sphinx. Only the project specific rules are supplied. The general
syntax is provided by the Sphinx documentation. Examples and templates
are provided to make creating documentation easier.

Change-Id: If97e1f01de5d4f3de4cad82b7b35279a199433bf
Signed-off-by: Rodrigo Caballero <rodrigo.caballero.abraham@intel.com>