Link support server in README

Too many people don't know that this exists. Especially considering how the docs are kinda lackluster, the support server is important to teach poise to people. So people should know it exists. Yeah.

README.md

@@ -12,7 +12,12 @@ Poise is an opinionated Discord bot framework with a few distinctive features:
 - text commands: commands are agnostic over old text-based commands and slash commands
 - edit tracking: when user edits their message, automatically update bot response
 
-# See [API documentation](https://docs.rs/poise/) and [examples](examples)
+# How to use
+
+Most information is in the [API documentation](https://docs.rs/poise/). Also take a look at the
+[examples](examples), especially `feature_showcase`, to learn what poise can do.
+
+For further questions, don't hesitate to join the support server: https://discord.gg/serenity-rs.
 
 # Bots using poise