novacustom/building-manual.md: note on building old releases

Signed-off-by: Filip Lewiński <filip.lewinski@3mdeb.com>

Author: filipleple

docs/unified/novacustom/building-manual.md

@@ -54,6 +54,12 @@
 
             > Replace `X.Y.Z` with a valid version
 
+            !!! warning
+
+                Releases predating ns5x_adl_v1.7.0, i.e. earlier than October 2023,
+                might not build in custom shells on OS'es other than Fedora. In such case,
+                it is recommended to run `bash` before `build.sh`.
+
             === "V540TU"
                 ```bash
                 git checkout novacustom_v540tu_vX.Y.Z