docs: Document romlib design

Change-Id: I2b75be16f452a8ab7c2445ccd519fb057a135812
Co-authored-by: John Tsichritzis <john.tsichritzis@arm.com>
Signed-off-by: John Tsichritzis <john.tsichritzis@arm.com>
diff --git a/docs/firmware-design.rst b/docs/firmware-design.rst
index 617cbb8..4f51ff1 100644
--- a/docs/firmware-design.rst
+++ b/docs/firmware-design.rst
@@ -1866,6 +1866,11 @@
                |   MHU    |
     0x04000000 +----------+
 
+Library at ROM
+---------------
+
+Please refer to the `ROMLIB Design`_ document.
+
 Firmware Image Package (FIP)
 ----------------------------
 
@@ -2662,5 +2667,6 @@
 .. _TF-A Interrupt Management Design guide: ./interrupt-framework-design.rst
 .. _Xlat_tables design: xlat-tables-lib-v2-design.rst
 .. _Exception Handling Framework: exception-handling.rst
+.. _ROMLIB Design: romlib-design.rst
 
 .. |Image 1| image:: diagrams/rt-svc-descs-layout.png?raw=true