Update WebSub documentation a bit (#5829)

* Update WebSub documentation a bit

* Marien prefers not to promote his hub too much :-)

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+# The WebSub protocol
+
+[WebSub](https://www.w3.org/TR/websub/) (formerly [PubSubHubbub](https://github.com/pubsubhubbub/PubSubHubbub)) is a standard protocol
+to instantly receive (push) notifications when some new content is available on a remote server,
+for instance when a new article is available via RSS / ATOM.
+
+FreshRSS supports WebSub natively.
+Just like for the FreshRSS API to work from a mobile phone, supporting WebSub requires that your FreshRSS instance is routable (that is to say, with a public IP, that can be accessed from third-party servers).
+
+## Examples of feeds
+
+Many individual feeds and platforms already offer instant notifications through WebSub, such as:
+[Friendica instances](https://friendi.ca), WordPress (from WordPress.com or with [an extension](https://wordpress.org/plugins/pubsubhubbub/)), Blogger sites, Medium sites, etc.
+
+## Test WebSub
+
+You can test that WebSub works properly in your FreshRSS instance with a service such as:
+
+* <http://push-tester.cweiske.de>
+
+## Add WebSub to your RSS / ATOM feeds
+
+Your CMS (e.g. WordPress) might already offer WebSub as an option. Otherwise, you can make a solution that notifies a hub such as:
+
+* <https://websubhub.com>
+* <https://pubsubhubbub.appspot.com>