Readme.md: Update instructions to fetch source

Openssl is fetched as a git submodule, so make sure git clone --recursive
is specified in the instructions to fetch a full source tree.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Nariman Poushin <nariman.poushin@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>