Update Mixin Documentation and Skeletor with a "How To"

[schristoff]

A tutorial that shows the user how to write a mixin would be great.
We should leverage existing documentation https://porter.sh/docs/how-to-guides/work-with-mixins/ , https://porter.sh/docs/development/dev-a-mixin/

[Pushkarmondal]

Hey @kichristensen @schristoff can I work on this?

[kichristensen]

@Pushkarmondal its would be awesome if you would take a look at this issue. Please reach out if you need any help, we are happy to help

[Pushkarmondal]

Sure

[lbergnehr]

Possibly out of scope, but take it for what it's worth: sometimes you cannot write your mixin using Skelletor, or in Go at all for that matter. In that case, it can be hard to understand what the mixin interface is. When I just started creating mixins I made a bash script that would simply print everything that Porter ran it with (parameters, stdin). I think it would be great to have such a "test" or "debug" mixin, or possibly just the documentation on a script that would do it and the documentation on how to install it.