Add contributing guidelines

[T0mstone]

As mentioned in #110 (comment).

These conventions are clearly contributing guidelines, so I went with CONTRIBUTING.md for the file name. That way we can also add other contributing guidelines in the future, if we want to.

If you want to reword something or add other conventions that I may have missed, don't hesitate to leave a comment.

From my side, there's only one unresolved question left here (which I also left as a TODO comment in the file itself), namely whether we want to write something about when we choose e.g. .t.r over .tr for corners (cf. #100).

[MDLC01]

My personal view is that this document should be used as a way to quickly find precedents, but that new names should always be motivated by existing symbols. In other words, those guidelines should reflect the existing conventions and shouldn't be used as a source of truth--the only source of truth is the existing names. For this reason, I strongly believe every rule in this document should be supported by one or multiple examples. This is reflected in my proposed rephrasing of the first paragraph.

Unrelatedly, if this document aims to be contributor guidelines, it might benefit from some of the links from the "Useful links" section of the proposals document. Maybe even a link to the document itself.

[T0mstone]

Another question that should be resolved: Should .cw and .ccw be generic modifiers instead?

Or, going further, perhaps the whole distinction should be removed entirely? (Tho I personally like it.)

[T0mstone]

Changing the title to reflect the broader scope.

[T0mstone]

Also, I forgot to say this earlier: Thanks for the thorough review!

[MDLC01]

Or, going further, perhaps the whole distinction should be removed entirely?

Maybe just ordering the modifiers from most concrete to least concrete would be enough. I personally don't see a clear cut distinction.

[MDLC01]

If you mean a clarification regarding corner delimiters, I don't think this is necessary.

[T0mstone]

Maybe just ordering the modifiers from most concrete to least concrete would be enough. I personally don't see a clear cut distinction.

Hmmm, I'd like to hear the other maintainers' opinions on this. Tho I'd definitely want to order them from least to most concrete instead.

[Enivex]

I think it would be helpful to have them organized alphabetically, at least within a category.

[T0mstone]

Another question that should be resolved: Should .cw and .ccw be generic modifiers instead?

I've now decided on "yes".

[T0mstone]

I think it would be helpful to have them organized alphabetically, at least within a category.

I kept some groups in the generic section that are still sort of ordered by importance, but the others are now sorted alphabetically.

[Enivex]

I think typst should be typeset in lowercase?

I'm wrong