Create a `CONTRIBUTING.md` guide

[AdrianGonz97]

It's not immediately obvious how someone could start contributing to this project, so we'll want to create a helpful guide to get people started.

At the very minimum, it should cover:

• building/running in dev
• how to test the cli locally
• how to run and write tests
• some style guidelines and code conventions

As an additional reference, here's the current CONTRIBUTING.md guide for Svelte

[benmccann]

Last I recall, the SvelteKit contributing guide might be a better one to follow though I haven't compared them lately

[manuel3108]

We might also be able to take some inspiration from svelte-add

[manuel3108]

Should also include details on how to run svelte-migrate locally, as pointed out by #494

[brettearle]

Can I put together a draft for the three of you to read through? As I have just trialled and errored my way through getting all this working locally over the last couple of sessions.
Happy to play scribe and do the writing polishing, I won't have the context though as I have just started getting my head around the codebase.

[manuel3108]

Absolutely, that would be fantastic!

[brettearle]

I'll get to in my next session