Docs: Added IAR specific build instructions

Added IAR specific build instructions tfm_build_instruction.rst

Added Note refering to tfm_build_instruction_iar.rst for IARARM builds

Change-Id: Ib0e6b5fd5039b544aafe2b81c03379cc9cf342a7
Signed-off-by: TTornblom <thomas.tornblom@iar.com>
diff --git a/docs/user_guides/tfm_build_instruction.rst b/docs/user_guides/tfm_build_instruction.rst
index f310bae..44f0b0a 100644
--- a/docs/user_guides/tfm_build_instruction.rst
+++ b/docs/user_guides/tfm_build_instruction.rst
@@ -44,6 +44,10 @@
    For building with Armclang compiler version 6.10.0, please follow the note
    in :ref:`docs/user_guides/tfm_sw_requirement:External dependencies` section.
 
+.. Note::
+   For building with the IAR toolchain, please see the notes in
+   :doc:`software requirements <tfm_build_instruction_iar>`
+
 Build steps for the AN521 target platform:
 ==========================================
 .. code-block:: bash