Readme.md: add submodule policy and clone commands

Readme.md: add submodule policy and clone commands

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1910

A section 'Submodules' is added to clarify the submodule policy
in edk2 repo. Git commands are also added to show the correct
way to clone submodule repos, in which '--recursive' is removed
because it's not needed but recommended in other document.

Related commits:
Openssl-1.1.1b upgrade: acfb90911840c38a0beb9bcfe0065668244d2b4d
berkeley-softfloat-3:   3cc57695df5a6e8c65fb46b993836c315cabf49d

Cc: Leif Lindholm <leif.lindholm@linaro.org>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Signed-off-by: Jian J Wang <jian.j.wang@intel.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Liming Gao <liming.gao@intel.com>