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: acfb909
berkeley-softfloat-3:   3cc5769

Cc: Leif Lindholm <[email protected]>
Cc: Michael D Kinney <[email protected]>
Cc: Liming Gao <[email protected]>
Signed-off-by: Jian J Wang <[email protected]>
Reviewed-by: Leif Lindholm <[email protected]>
Reviewed-by: Liming Gao <[email protected]>

Readme.md

@@ -140,3 +140,42 @@ Signed-off-by: Contributor Name <[email protected]>
   the change.  Each line should be less than ~70 characters.
 * `Signed-off-by` is the contributor's signature identifying them
   by their real/legal name and their email address.
+
+# Submodules
+
+Submodule in EDK II is allowed but submodule chain should be avoided
+as possible as we can. Currently EDK II contains two submodules
+
+- CryptoPkg/Library/OpensslLib/openssl
+- ArmPkg/Library/ArmSoftFloatLib/berkeley-softfloat-3
+
+The latter one is actually required by previous one. It's inevitable
+in openssl-1.1.1 (since stable201905) for floating point parameter
+conversion, but should be dropped once there's no such need in future
+release of openssl.
+
+To get a full, buildable EDK II repository, use following steps of git
+command
+
+```
+$ git clone https://github.com/tianocore/edk2.git
+$ cd edk2
+$ git submodule update --init
+$ cd ..
+```
+
+If there's update for submodules, use following git commands to get the
+latest submodules code.
+
+```
+$ cd edk2
+$ git pull
+$ git submodule update
+```
+
+Note: When cloning submodule repos, '--recursive' option is not
+recommended. EDK II itself will not use any code/feature from
+submodules in above submodules. So using '--recursive' adds a
+dependency on being able to reach servers we do not actually want
+any code from, as well as needlessly downloading code we will not
+use.