[otbn] Add a pointer to the OTBN software guide in otbn/README.md

Signed-off-by: Rupert Swarbrick <rswarbrick@lowrisc.org>

hw/ip/otbn/README.md

diff --git a/hw/ip/otbn/README.md b/hw/ip/otbn/README.md
index af52460..4d7ff01 100644
--- a/hw/ip/otbn/README.md
+++ b/hw/ip/otbn/README.md
@@ -22,6 +22,8 @@
 `hw/ip/otbn/util`. These are wrappers around a RISC-V GCC and binutils toolchain
 so one must be available (see the OpenTitan documentation on [obtaining a
 toolchain](https://docs.opentitan.org/doc/ug/install_instructions/#software-development).
+For more details about the toolchain, see the [user
+guide](https://docs.opentitan.org/doc/ug/otbn_sw)).
 
 `hw/ip/otbn/util/build.sh` provides a simple script to build a single OTBN
 assembly files using the toolchain: