sel4test+wrapper: add comment explaining why --release is forced

Change-Id: I1b50e8c65f4b893788ee6d87f18f60549801bed5

integrations/sel4-sys-wrapper/CMakeLists.txt

diff --git a/integrations/sel4-sys-wrapper/CMakeLists.txt b/integrations/sel4-sys-wrapper/CMakeLists.txt
index 21cd8a4..1d8c767 100644
--- a/integrations/sel4-sys-wrapper/CMakeLists.txt
+++ b/integrations/sel4-sys-wrapper/CMakeLists.txt
@@ -20,6 +20,8 @@
 #  -DRustCFlags="" \
 #  -DRustVersion=nightly-2021-11-05
 
+# NB: force --release; debug builds do not work (they run out of memory)
+
 add_custom_command(
     OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/target/libsel4_sys.a
     COMMAND ${CMAKE_COMMAND}