I had a spare couple of minutes and there was a new LJ release email in my inbox and so I found it for you by looking at previous release posts in lj_releases (which is worth watching for just these reasons).
The <lj-like> tag is mentioned in release #93 from June 19 2012.
The lj-like FAQ page.
LJ has a like button
Happy Birthday elyssa! I use the LJ notification emails to remind me to make these posts and so I apologise if I miss anyone's…