libusbg: doc: Add document about gadget schemes
libusbg: doc: Add document about gadget schemes

Add document which clarify reasons of implementing
gadget schemes and also describes syntax of such files.

Signed-off-by: Krzysztof Opasiak <8064eda6f94b3817252b7d7649cf754592f63500@samsung.com>

pszewczyk/libusbg,pszewczyk/libusbg