mktmpdir: refer to docs

Manual BUILDDIR specification isn't exactly obvious
from this script (aiming to derive it automatically),
so at least point those interested at QUICKSTART
they might have long forgotten already or, gasp,
not even started reading.

bin/mktmpdir

@@ -2,6 +2,8 @@
 # analyze free space, preferring tmpfs over really many gigaz
 # and taking into account configured hasher workdir prefices
 
+# NB: use BUILDDIR to override autodetection (see ../QUICKSTART)
+
 # hope there aren't spaces in RM's $HOME are they?
 DIRS="$TMP $TMPDIR $HOME/hasher /tmp /var/tmp .."
 MINSIZE=262144		# face control criterion