Add contributing guidelines

CONTRIBUTING.md

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+# Contributing Guidelines
+Contributions are welcome! This document provides some resources and guidelines to help with the process.
+
+Symbol definitions are in `src/modules/`; The syntax should be mostly self-evident.
+If you need help with a contribution, you can ask us [on Discord](https://discord.com/channels/1054443721975922748/1277628305142452306).
+
+## Proposals
+Proposals used to be written in the [Proposals document](https://typst.app/project/riXtMSim5zLCo7DWngIFbT),
+although it is now preferred to have a GitHub issue for each one.
+Nonetheless, the document still contains a lot of useful information.
+
+## Conventions
+When adding new modules, symbols or variants, please try to be consistent with
+existing ones. Below are some guidelines based on existing symbols. These aren't
+always hard rules, especially because of how messy Unicode can be, but you should
+adhere to them if possible.
+
+### General Conventions
+- Modifier and module names are entirely lowercase.
+- Symbol names are lowercase unless the symbol is an uppercase letter.
+- Symbol names should be at least two characters long so they can be used easily in Typst's math mode.
+- When a symbol is added to a base, the symbol name is used as a modifier on the base.[^modifname]
+	This can have the following meanings:
+	- The symbol is added around or inside the base as a subordinate (smaller than the base),
+		e.g. `eq.quest`, `triangle.stroked.dot`.
+	- The symbol is stacked below the base, e.g. `gt.lt`.
+	- The symbol is stacked to the right of the base, e.g. `colon.eq`.
+	- The symbol is overlaid at the center of the base, e.g. `integral.dash`.
+	- The symbol surrounds the base, e.g. `plus.square`.
+- Notable exceptions to the previous convention:
+	- When `.eq` is used in the second sense (stacked below), it only adds a single line and not two,
+		e.g. `lt.eq`. For two lines below, `.equiv` is used, e.g. `lt.equiv`.
+
+[^modifname]: Though a modifier can also just coincidentally be a symbol name, e.g. `.not`.
+
+### Established Generic Modifiers
+These have a broad meaning and can have varying interpretations.
+<!-- Geometry -->
+- `.l`/`.r`/`.t`/`.b`: The four main directions (left/right/top/bottom), e.g. `arrow.l`, `times.r`.
+	- For delimiters, `.l` means opening and `.r` means closing, e.g. `paren.l`, `quote.r`.
+- `.tl`/`.tr`/`.bl`/`.br`: The four corners, e.g. `arrow.tl`, `triangle.stroked.tr`.
+	- Generally, these are used for a single, diagonal direction,
+		whereas combinations of two main directions (like `.t.l`) are used to mean both of them at once,
+		e.g. `arrow.t.l`, if it existed, would be an arrow that points both top and left,
+		similarly to how `arrow.l.r` is an arrow pointing both left and right.
+- `.cw`/`.ccw`: Clockwise/Counterclockwise, e.g. `arrow.cw`, `integral.ccw`.
+- `.tiny`/`.small`/`.medium`/`.big`: A geometric shape with a certain size, e.g. `square.stroked.small`.
+<!-- Strokes -->
+- `.stroked`/`.filled`: A symbol that has an empty/filled interior, e.g. `circle.stroked`, `arrow.r.filled`.
+	(They correspond to Unicode's "white"/"black".)
+- `.dotted`: A shape with a dotted line instead of a full stroke, e.g. `circle.dotted`.
+- `.light`/`.heavy`: A shape with a certain stroke weight, e.g. `checkmark.heavy`.
+<!-- Other (in alphabetic order) -->
+- `.alt`: An alternate glyph for the symbol, e.g. `phi.alt`.
+- `.double`, `.triple`, `.quad`: A symbol that has 2-4 of something, e.g. `excl.double`, `eq.quad`.
+
+### Established Concrete Modifiers
+These have a specific meaning that is not open to much interpretation.
+<!-- (in alphabetic order) -->
+- `.big`: A [large](https://www.unicode.org/Public/math/latest/MathClassEx-15.html) (n-ary) version
+	of an operator, e.g. `union.big`.
+- `.inv`: Either vertically mirrored or a 180° rotated version of a symbol, e.g. `amp.inv`, `Omega.inv`.
+	- See also [#108](https://github.com/typst/codex/issues/108).
+- `.not`: A negation of the symbol, e.g. `eq.not`.
+- `.o`: A symbol with a circle around it, e.g. `plus.circle`.
+	- See also [#62](https://github.com/typst/codex/pull/62)
+- `.rev`: A horizontally mirrored version of a symbol, e.g. `in.rev`.
+	- See also [#108](https://github.com/typst/codex/issues/108).
+- `.sq`: A "squarified" version of a symbol, e.g. `subset.sq`.
+	- See also [#110](https://github.com/typst/codex/pull/110)