Add CONTRIBUTING.md file

[dmint789]

This PR adds a CONTRIBUTING.md file, which outlines a bunch of guidelines for contributing to this repo. This was discussed in #130. The digits mentioned in the document would look like this:

The main reason I mentioned the digits was to enable the addition of icons for 8x8+ puzzles (#125).

According to the naming guidelines in this file, the code changes below should be made. I believe I am the only user of all of these, since I created all of these icons, and it hasn't been that long since then, so there shouldn't be any other affected users. And even if there are other users, they'd still be using the old package version with the icon codes their code expects.

333_oh_bld_team_relay -> 333_oh_bld_3p_relay
333bf_2_person_relay -> 333bf_2p_relay
333bf_3_person_relay -> 333bf_3p_relay
333bf_4_person_relay -> 333bf_4p_relay
333bf_8_person_relay -> 333bf_8p_relay
miniguild_2_person -> miniguild_2p

I'd also like to change both uses of mirror_blocks to simply mirror, but I understand that that might be a step too far for some contributors. Just mentioning it here in case someone agrees with it.

[dmint789]

To-do:

• Add point about an official event becoming unofficial too
• Add guideline for where the competitors (circles) should be positioned
• Add guideline about one-handed events having the puzzle above the hand shape

[dmint789]

@jfly any thoughts on this?

[dmint789]

@jfly kind ping :)

[jfly]

@lgarron and I will talk about once we've got our heads out of twsearch land, hopefully in a month or two.

Until then, I'm ok adding more unofficial icons.