Document how to set up build users on Mac OS X

doc/manual/installation/multi-user.xml

@@ -52,6 +52,34 @@ This creates 10 build users. There can never be more concurrent builds
 than the number of build users, so you may want to increase this if
 you expect to do many builds at the same time.</para>
 
+<para>On Mac OS X, you can create the required group and users by
+running the following script:
+
+<programlisting>
+#! /bin/bash -e
+
+dseditgroup -o create nixbld -q
+
+gid=$(dscl . -read /Groups/nixbld | awk '($1 == "PrimaryGroupID:") {print $2 }')
+
+echo "created nixbld group with gid $gid"
+
+for i in $(seq 1 10); do
+    user=/Users/nixbld$i
+    uid="$((30000 + $i))"
+    dscl . create $user
+    dscl . create $user RealName "Nix build user $i"
+    dscl . create $user PrimaryGroupID "$gid"
+    dscl . create $user UserShell /usr/bin/false
+    dscl . create $user NFSHomeDirectory /var/empty
+    dscl . create $user UniqueID "$uid"
+    dseditgroup -o edit -a nixbld$i -t user nixbld
+    echo "created nixbld$i user with uid $uid"
+done
+</programlisting>
+
+</para>
+
 </simplesect>