Source for upcoming rule changes
The official announcement of any rules changes is made in an article on Wizards' Magic site. This article is generally posted the week of a set's pre-release event (Khans of Tarkit Update Bulletin), or in the case of products that do not have pre-releases like the Commander 2014 decks the set's release date. Occasionally they are a little late for example the Commander 2014 Release Notes were not posted until the week after the decks released.

Wizards also usually release an article when previews start detailing all of the mechanics that will be in the set, for example Mechanics of Khans of Tarkir. The difference between these articles is that the Mechanics of ______ articles do not have any actual rules text, they just describe how the mechanics work, and they only contain information on one set. 

The Update Bulletin on the other hand has the actual text for the mechanics that will be in the Comprehensive Rules, it also contains any other updates and changes they are making to other rules (for example they recently changed Convoke and Delve in the Magic 2014 Update Bulletin even though Delve wasn't in the set at all). It also contains any changes to the Oracle text of cards. 

For your Judge Cast video Wizards actually posted an article about a month prior to the video listing some of the changes they were planning on making. This doesn't happen very often, but they do it occasionally for bigger changes that might change how people play the game. So for that change there was a big change to how the Legend Rule and Planeswalker Uniqueness Rules worked and releasing the news early allowed people to prepare for the changes, so for example if you played Commander and your decks way of dealing with opposing commanders was to clone them that would no longer work so you would have to redo a large portion of your deck and telling everyone there would be changes coming would allow you to get your deck ready for when they did.

So while they occasionally post additional articles on their site, the primary source for any rules information is usually either the Mechanics of ______ and the ______ Update Bulletin articles.

Anything outside of an official Wizards source is a secondary source probably getting their information from either of these two sources, or statements by Wizards employees on social media. On rare occasions there might be leaks of upcoming rules that other sites report on before it is officially posted on Magic's site I would not consider them to be a primary source, since they are still getting their information from Wizards it just isn't published yet.