the file docs/filter-howto.txt was removed, as it contained misleading
information since it was first checked in.
it suggests the syntax for filter rules is fnmatch()-like, when in
fact they need to be specified as posix regular expressions.
additionally it contained a lot of utterly unrelated and irrelevant/
outdated text.
a few examples with the correct syntax have now been added to
tinyproxy.conf.5 manpage.
closes#212
