brew installed software (keg-only) which includes /usr/local/Cellar/*/share/man/ - No manual entry for readline
The -M option for man allows you to specify a path. The exact command you're looking for is:

man -M 3 /usr/local/Cellar/readline/7.0.1/share/man/man3/readline.3


Note that the number after the -M specifies the "man page section", which is the last character in the file path. For example, I have two versions of sshd installed and I read can both their man pages with the following commands:

man -M 8 /usr/share/man/man8/sshd.8
man -M 8 /usr/local/Cellar/openssh/7.3p1/share/man/man8/sshd.8


I'm curious why brew didn't symlink readline, since brew's readline is the only one installed - presumably past versions (I'm on Sierra) of macOS did have readline? I'd tend to be conservative here, and honour brew's decision as far as possible - just in case Apple decide to install readline again with a future OS upgrade. That being said, you could manually symlink readline:

ln -s /usr/local/Cellar/readline/7.0.1/share/man/man3/readline.3 /usr/local/share/man/man3/readline.3


The risk with this is that - if Apple do install readline in the future - you'll end up with Apple's readline in your $PATH (and not brew's, since it's keg-only and hasn't been linked), but brew's readline will occur first in your $MANPATH, before the man page for Apple's readline.