README: minor fixes
README: minor fixes

Add additional pre-requisites
Minor fixes of table of contents

Signed-off-by: Igor Opaniuk <ca782e5848fbe7ab0a250faa8ae33b643dd89a09@linaro.org>
Reviewed-by: Joakim Bech <741f90e2a7f4d9afbad8bd50be20560f4cf38962@linaro.org>

pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os