Follow symlinks in search of doc directories and exclude duplicate ~/.vim/doc directory in dir list

Initially, without reading the source thoroughly, I declared `~/.vim/doc` in the `dir` list resulting in duplicate completions, hence the remark in `README.md` and a duplicate check/removal implementation for convenience.

Second, my `~/.vim/bundle/` holds symlinks to "real" directories that are not searched in without the `-L` switch for `find` to follow symlinks.