Repo: Organise and document processes and architecture (#43)

* Add Makefile for common contributor tasks

Narrows down a few commands to automatically format when building, and neatly expose testing/benching.

Empty files added to describe contributor guidelines, overall architecture.

* First draft of contributor guidelines

* Simple architecture diagrams

* Add PNG variants of architecture diagrams

Swapping to these because not having Fira Sans installed on a viewing machine leads to terrible kerning.

* Architecture description.

* MD cross-refs.

README.md

@@ -20,7 +20,6 @@ The library offers:
 Songbird's gateway functionality requires you to specify the `GUILD_VOICE_STATES` intent.
 
 ## Dependencies
-
 Songbird needs a few system dependencies before you can use it.
 
 - Opus - Audio codec that Discord uses.
@@ -45,8 +44,10 @@ This is an optional dependency. It allows Songbird to download an audio source f
 ## Examples
 Full examples showing various types of functionality and integrations can be found in [this crate's examples directory].
 
-## Attribution
+## Contributing
+If you want to help out or file an issue, please look over [our contributor guidelines]!
 
+## Attribution
 Songbird's logo is based upon the copyright-free image ["Black-Capped Chickadee"] by George Gorgas White.
 
 [serenity]: https://github.com/serenity-rs/serenity
@@ -54,6 +55,7 @@ Songbird's logo is based upon the copyright-free image ["Black-Capped Chickadee"
 ["Black-Capped Chickadee"]: https://www.oldbookillustrations.com/illustrations/black-capped-chickadee/
 [lavalink]: https://github.com/Frederikam/Lavalink
 [this crate's examples directory]: https://github.com/serenity-rs/songbird/tree/current/examples
+[our contributor guidelines]: CONTRIBUTING.md
 
 [build badge]: https://img.shields.io/github/workflow/status/serenity-rs/songbird/CI?style=flat-square
 [build]: https://github.com/serenity-rs/songbird/actions