Add RSS feed with alternatives

[LukasReschke]

Ref nextcloud/security-advisories#25

@jospoortvliet WDYT? It's the old feed and a new entry:

We have moved our new security advisories to GitHub Security Advisories. If you rely on a RSS feed, you can find alternatives discussed in nextcloud/security-advisories#25.

[m-ueberall]

Would it be possible to simply redirect the old URI to the feed mentioned in nextcloud/security-advisories#25? This should minimize the required work on user side. (If I'm not mistaken, in this way, users should temporarily see some duplicates in the worst case.)

[esbeeb]

I too would strongly prefer to receive Security Advisories over RSS, and it would be nice if no 3rd-party services were needed to make this happen (which can become a point of failure in and of itself, should the RSS feed stop working, as was the case for the feed mentioned in nextcloud/security-advisories#25)

Just to highlight that this is not an unreasonable thing to have, Gitlab, by comparison, makes a convenient RSS icon to subscribe to lists of issues. An example (albeit, those are not security advisories) is seen here, which I happened across the other day. Look in the upper right corner to see the RSS icon.