Docs: Add a note to avoid using GNU Arm 10-2020-q4-major

Add a note to avoid using GNU Arm compiler version 10-2020-q4-major. It
has a bug in CMSE support, which will break CMSE functinalities.

Change-Id: If1cbc6582f2b36ed990dc737eaa67b2ef2f888e8
Signed-off-by: David Hu <david.hu@arm.com>
diff --git a/docs/getting_started/tfm_sw_requirement.rst b/docs/getting_started/tfm_sw_requirement.rst
index 530f7f9..4a30aa9 100644
--- a/docs/getting_started/tfm_sw_requirement.rst
+++ b/docs/getting_started/tfm_sw_requirement.rst
@@ -51,6 +51,10 @@
       On the page select *GNU Arm Embedded Toolchain: 7-2018-q2-update* or an
       above version.
 
+    - GNU Arm compiler version *10-2020-q4-major* has an issue in CMSE support.
+      The bug is reported in `here <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99157>`__.
+      Select other GNU Arm compiler versions instead.
+
     - The IAR Arm compilers above are provided via IAR Embedded Workbench (Windows) or
       IAR build tools for linux.
       For information, see