docs: Add explicit reference to unprivileged userns docs of mkosi

Fixes #40216

diff --git a/docs/HACKING.md b/docs/HACKING.md
index aa226445e7b1d413ddb1d44a065dee44794081ed..f2c21cb16ec6b2584bac8f7df11abb3e5dbea44c 100644 (file)
--- a/docs/HACKING.md
+++ b/docs/HACKING.md
@@ -36,6 +36,10 @@ or via your distribution's package manager. Note that systemd regularly adopts
 newer mkosi features that are not in an official release yet so there's a good
 chance that your distribution's packaged version of mkosi will be too old.
 
+Make sure to read the "Unprivileged User Namespaces" section in the mkosi documentation
+(run `mkosi documentation` to view the mkosi docs) and apply any necessary instructions
+to make sure unprivileged user namespaces work on your system.
+
 Then, you can build, run and test systemd executables as follows:
 
 ```sh