chore: add contribution guide

[SethFalco]

Adds a contributing guide. I'll improve this over time, but there were some highlights I wanted to put out already.

• Most importantly, to ask that contributors pair their changes with documentation if appropriate.
• To direct contributors to the Plugins Architecture documentation.
• On the side, that users with questions could consider if it's worth asking it on Stack Overflow rather than an issue. (Since that's a much more active community for support and keeps things tidier here. I personally watch the [svgo] tag too.)

The rest was me adding context and trying to make contributing to SVGO beginner-friendly.