Add notes about performance and lock disabing

src/imapdomo.skel

@@ -16,13 +16,19 @@ usage() {
             "-c DIR      change to DIR before doing anything"                 \
             "-l          list handlers and exit"                              \
             "-d          turn on debugging mode"                              \
-            "-N          turn off locking"                                    \
+            "-N          turn off locking; make sure that handlers cannot"    \
+            "            interfere with each other or that another locking"   \
+            "            mechanism is in place.  Also see NOTE below."        \
        --                                                                     \
        "imapdomo will try to read init.lua and handlers/ACTION.lua from its"  \
        "configuration directory."                                             \
        ""                                                                     \
        "See imapfilter_config(5)) for guide and API reference.  Few functions"\
-       "are also available in .imapdomo/common.lua"
+       "are also available in .imapdomo/common.lua"                           \
+       ""                                                                     \
+       "NOTE: Be aware that it's your responsibility to ensure that filters"  \
+       "don't cause performance problems. (Trust me, it's not so hard to"     \
+       "\"earn\" yourself a nice server ban--I learned that the hard way)."
 }
 
 bug() {