See the announcement on Reddit for more details, or go direct to the release page on GitHub.
EDIT If you've already downloaded alpha 5, then please go back to the above page to download alpha 5.1 - a hotfix that allows your psi operatives to gain promotions at a normal rate again. Otherwise your psi operatives will be pretty useless.
Alpha 5 released!
Re: Alpha 5 released!
Godspeed, guys! Russia approves and apriciates your efforts!