Add contributing guide to STMicroelectronics/optee_os repository.

Change-Id: If81324a6df8fd08f5a2c75235513467124ea44e3
Signed-off-by: Bernard Puel <bernard.puel@st.com>
Signed-off-by: Lionel Debieve <lionel.debieve@st.com>
Signed-off-by: Etienne Carriere <etienne.carriere@st.com>