This package makes it possible to use proselint with Emacs built-in Flymake.
flymake-proselint is listed on GNU
ELPA, so you can use
M-x list-packages to install it.
Then you just need to activate it in the modes you want your prose to be checked with something like:
(add-hook 'text-mode-hook (lambda () (flymake-mode) (flymake-proselint-setup)))