[PATCH v2 4/4] doc: Describe new compat arch support

[Jan Kiszka <jan.kiszka@siemens.com>]

From: Jan Kiszka <jan.kiszka@siemens.com>

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
---
 doc/user_manual.md | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/doc/user_manual.md b/doc/user_manual.md
index 9c28d165..60802dc6 100644
--- a/doc/user_manual.md
+++ b/doc/user_manual.md
@@ -723,6 +723,19 @@ Debian cross-compilation works out of the box starting from Debian stretch distr
 
 Experimental support for riscv64 is available as well.
 
+### Cross-building for a compat architecture
+
+Some architectures, under Isar amd64 and arm64 so far, support running 32-bit
+legacy applications on 64-bit kernels. Debian supports this via the multiarch
+concept.
+
+Isar can build 32-bit packages as part of a 64-bit image build and also enable
+the image with the necessary packages. To activate the compat mode of a build,
+set `ISAR_ENABLE_COMPAT_ARCH = "1"` in `local.conf`. Packages that shall be
+built for the compat arch need to be tagged individually by setting
+`PACKAGE_ARCH = "${COMPAT_DISTRO_ARCH}"` in the package recipe. Non-tagged
+packages will continue to be built for the primary target architecture.
+
 
 ## Examining and debugging package generation inside their buildchroot
 
-- 
2.26.2