README: update documentation wrt. build dependencies
README: update documentation wrt. build dependencies

Explain how optional dependencies can be avoided.

Signed-off-by: David Wagner <60228a510f9a76940848fa2d29321fc4edfc2119@intel.com>

mgaio/parameter-framework,01org/parameter-framework,miguelgaio/parameter-framework,01org/parameter-framework,mgaio/parameter-framework,mgaio/parameter-framework,dawagner/parameter-framework,miguelgaio/parameter-framework,01org/parameter-framework,dawagner/parameter-framework,dawagner/parameter-framework,miguelgaio/parameter-framework