[PATCH v3 1/2] Readme.md: add submodule policy and clone commands

["Wang, Jian J" <jian.j.wang@intel.com>]

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>
---
 Readme.md | 39 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 39 insertions(+)

diff --git a/Readme.md b/Readme.md
index 7c873759a5..27e4ce0771 100644
--- a/Readme.md
+++ b/Readme.md
@@ -140,3 +140,42 @@ Signed-off-by: Contributor Name <contributor@example.com>
   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.
-- 
2.17.1.windows.2