Mention how to add RSS feeds on different channels

[RJ722]

We need to document the behavior of RSS App somewhere on whattodo.readthedocs.io:

Something along the lines of:

RSS feeds are hugely helpful for following a blog or a podcast. AMU-OSS's slack channel has the RSS app installed which will post a new message on slack whenever there's an update on the feed.

The following commands can be used to manage the RSS feeds linked in any slack channel:

/feed subscribe <link_to_RSS_feeds>
/feed list
/feed help

These commands can be typed into any slack channel.