Honestly the fastest way to know about what is going on around would be to be present in the Developer`s channel on https://pre-release.mattermost.com/ however the twitter https://twitter.com/mattermosthq seems pretty close(doesn’t list RC2 yet) to the latest news about it, but there is a lot of other stuff around in it.
If you follow the release page of mattermost on github you will know everything as well https://github.com/mattermost/platform/releases like RC3 which was posted 3 hours ago.
To explain a bit for those not aware, RC are release candidates, which are BETA that have potential as final product, unless substantial bugs are found.
I’ve asked @lindy about it, she said that the newsletter will mostly tell about releases and security patches, regards your goal.
I’ve also quoted them on what would be the best place to get notified about what you want and will update here once they reply, or perhaps they will reply directly instead 
Hopefully MM staff might provide a better information on that, that I can provide you with.
The blog also have a feed, its a bit hidden but here is an example: https://about.mattermost.com/category/blog/feed/