According to Musk, securing it turned out not to be necessary:

https://twitter.com/elonmusk/status/726218218109444096

Early plan was to weld shoes over the feet. https://www.reddit.com/r/IAmA/comments/2rgsan/i_am_elon_musk...

For CRS-8 they used jacks and chained it hooks welded on the deck. Done by humans, not remotely.

https://i.imgur.com/NAJU4JS.jpg

https://i.imgur.com/IBZmewR.jpg

Not sure if they'll keep doing this, given Musk's tweet saying it doesn't need securing. https://twitter.com/elonmusk/status/726218218109444096

They weld a metal shoe over each landing leg.

I believe it rests under its own weight and friction. In an interview, Musk said the acceptable wave height was 2-3 times higher than the ones seen during that landing.

I'm a little skeptical about that. You can clearly see the rocket slide as it lands.

It still has surplus horizontal velocity when it hits the deck, hence the sliding. [1] Musk confirmed on Twitter that the rocket in fact doesn't need securing and the metal shoe plan turned out to be unnecessary.

[1]: https://twitter.com/elonmusk/status/726218218109444096

I wonder how they determined that it's unnecessary. Did they try to topple it over?

Really heavy objects with a low center of mass don't just topple over. It's 42 meters tall and weighs around 20.000 kg without fuel.

http://spaceflight101.com/spacerockets/falcon-9-v1-1-f9r/