Added doc on Makie package extension

[kapple19]

Description

Fixes lack of documentation on adding a package extension for Makie.

Type of change

Appended package extension implementation guide to recipes.md.

Checklist

• Added an entry in CHANGELOG.md (for new features and breaking changes)
• Added or changed relevant sections in the documentation
• Added unit tests for new algorithms, conversion methods, etc.
• Added reference image tests for new plotting functions, recipes, visual options, etc.

[kapple19]

@jkrumbiegel I'm not sure if you preferred I don't link to your example.
If not, let me know and I can remove the link,
but also add more of the example from your code into the documentation here.

Also let me know if you'd like more details or to actually add an example.

[jkrumbiegel]

What I had been thinking about was to add an extension test to our test suite, and this would double as an example to point people to. Being a test, it would not go out of sync and we could be sure that our intended extension mechanism keeps working in future julia versions.

[SimonDanisch]

@kapple19 can you add a changelog entry? :)
https://github.com/MakieOrg/Makie.jl/blob/master/CHANGELOG.md

[jkrumbiegel]

@SimonDanisch I would say a docs change doesn't need one, I'll use the skip label here