ispell private dictionary per directory

[kgold]

How can I tell emacs/ispell to insert and use a private dictionary
located in the current directory?  I find that different projects
(source code, documentation, etc.) have different private words.

I know that I can create a private ispell dictionary, but emacs by
default creates a global one in the home directory.  I also see
ispell-dictionary-alist, to create a list of dictionaries, but that
also seems to be fixed and global.

ispell by default looks for a personal dictionary in the current
directory as well as the home directory, so the feature seems to be
there.  But there's no obvious way, in ispell or emacs, to insert
words into the current directory's private dictionary.