Add explanation of CA certificate storage logic

src/imapdomo.skel

@@ -29,6 +29,12 @@ usage() {
        "See imapfilter_config(5)) for guide and API reference.  Few functions"\
        "are also available in .imapdomo/imapdomo.lua"                         \
        ""                                                                     \
+       "By default, imapdomo will look for 'certificates' file under the"     \
+       "directory (__IMAPDOMO_CONFIG_USER__), and if it exists, it will tell" \
+       "imapfilter to use it via -t argument.  If it does not exist, it will" \
+       "leave the argument out, meaning imapfilter will use system cert"      \
+       "storage.  Use -T to force system storage or -t to force another file."\
+       ""                                                                     \
        "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)."