README: clarify that not only systemd should use prefix=/usr, but its deps too

See: #9547

diff --git a/README b/README
index 61a1fd45bfe1c3536eb841ff4e74d50755ade6ad..82f47a59ab6cfbcf40bbab58ca1e87568e3d4b04 100644 (file)
--- a/README
+++ b/README
@@ -205,10 +205,12 @@ REQUIREMENTS:
         make use of DynamicUser= now, hence enabling nss-systemd is not
         optional.
 
-        Note that the build prefix for systemd must be /usr. -Dsplit-usr=false
-        (which is the default and does not need to be specified) is the
-        recommended setting, and -Dsplit-usr=true should be used on systems
-        which have /usr on a separate partition.
+        Note that the build prefix for systemd must be /usr. (Moreover,
+        packages systemd relies on — such as D-Bus — really should use the same
+        prefix, otherwise you are on your own.) -Dsplit-usr=false (which is the
+        default and does not need to be specified) is the recommended setting,
+        and -Dsplit-usr=true should be used on systems which have /usr on a
+        separate partition.
 
         Additional packages are necessary to run some tests:
         - busybox            (used by test/TEST-13-NSPAWN-SMOKE)