Add instructions to the README explaining how we want people to consume SVGs

[jfly]

We aren't exactly sure how we want people to consume our SVGs. Are they an implementation detail? Should we include them in our NPM package?

[lgarron]

I've left a comment about #151 (comment) with a bit more info.

I think until we have a conclusion at that discussion, pointing people at the repo as a "here be dragons, but you can be reasonably assured we won't change things willy-nilly" is best.

For example, it's not too difficult to download and vendor based on one of these URLs:

• https://api.github.com/repos/cubing/icons/zipball/main
• https://api.github.com/repos/cubing/icons/zipball/v3.0.5
• https://api.github.com/repos/cubing/icons/releases/latest (e.g. the "zipball" field)

(Also, git submodules would work but I would never recommend that.)

If we add something to the README, I'd also like to encourage people to comment an issue with their use case, so we know what to optimize for when we do add SVG export.