It appeared first on 4chan, and was reposted roughly simultaneously by accounts with old registration dates/times but no posts on several other community forums. I think it is legitimate, was intentionally released by GW (though they'll deny it if we ask), and we are meant to discuss it and attempt to "break" it to appease/help our GW Overlords in making a quality product.
People on another forum that is named after the word that Orks use for machine guns firing claim that /tg/ made the whole thing up, but honestly, even /tg/ isn't that organized. And it's not April 1st.
Some highlights:
Wound allocation abuse is fixed in two ways -- wounds are allocated based upon save values instead of wargear, and the Patch Up consolidation which means multiwound units have to shift wounds around even if means abandoning injured soldiers (but you also get to heal some multiwound models!)
You can use Flamers to shoot into transports, which fixes IG Meltavet Chimera Spam.
Monstrous Creatures are a lot harder to tarpit.
Vehicles are much easier to hit.
Rail weapons use the "line" effect of Jaws of the World Wolf and the Doomscythe's Death Ray.
The Sonic Weapons used by Noise Marines are Rail type weapons.
You can use pistols in Assault with a different profile.
Assault Phase now comes before Shooting Phase.
There is an Assault Move that can made immediately following a Deep Strike, but it doesn't get Charge bonuses (this explains some of the weirdness with Necron CC units)
Power Weapons with no other special powers confer a 5++ "parry" invuln save.
Overwatch is back!
If a unit Deep Strikes or Infiltrates within 12" of a unit, the unit that's about to get assaulted gets to make a Defensive Shot against the Deep Striking unit.
Deep Strikers do not scatter if they are more than 18" away from an enemy. This, as you might imagine, fixes a lot that's bad about Codex Chaos Daemons, and makes Deep Strike flyers such as Valkyries and Stormravens a lot more fun.