Readme.md: Change way of cloning edk2 repo

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



'--recursive' in cloning edk2 repo might cause failure on machine
behind firewall. It's not actually necessary for edk2. This patch
removed it from git command. A separate command

   $ git submodule update --init

is added to retrieve repo specified via submodule.

Cc: Leif Lindholm <leif.lindholm@linaro.org>
Cc: Ard Biesheuvel <ard.biesheuvel@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>