FS#1750 - Provide notification channel for release announcements

[openwrt-bot]

azrdev:

After installing OpenWrt on any device, I have to actively monitor the project website to know whenever a new release is available, potentially fixing security issues with my installation.

I propose creating a release-announce mailinglist used only for that purpose, readable & subscribable for everyone.

There might already be other channels offering more push-like notifications, but I didn't find any ones that are officially sanctioned thus reliably complete.