Add simple docs for userdev

[Machine-Maker]

Partially addresses #19

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First crack at better docs than just linking to the test plugin.

[MiniDigger]

good start, maybe a section about the tasks that you wanna use?
and maybe for finishing up a screenshot of accessing internals?

[powercasgamer]

This should probably also mention what jar to actually use, see a lot of people that use the -dev or -dev-all jars

[olijeffers0n]

Obviously Still a draft PR, but TBH looks good apart from these comments