Add contributing guidelines

CONTRIBUTING.md

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+# Conventions
+When adding new modules, symbols or variants, please try to be consistent with
+the following list of conventions. These aren't always hard rules, especially
+because of how messy Unicode can be, but you should adhere to them if possible.
+
+General conventions:
+- Modifiers are entirely lowercase.
+- Modifiers that correspond to symbols (by having the same name) have six established meanings:
+	- The symbol is added around the base like an accent or an attachment (smaller than the base), e.g. `eq.quest`.
+	- 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`.
+	- The symbol is placed inside the base, e.g. `triangle.stroked.dot`.
+Established generic modifiers:
+- `.l`/`.r`/`.t`/`.b`: The four main directions (left/right/top/bottom).
+	- For delimiters, `.l` means opening and `.r` means closing (see [#100](https://github.com/typst/codex/pull/100)).
+- `.tl`/`.tr`/`.bl`/`.br`: The four corners
+	<!-- TODO: Do we have or want to have conventions about when to choose `.tl` vs. `.t.l`? -->
+- `.double`, `.triple`, `.quad`: A symbol that has 2-4 of something.
+- `.stroked`/`.filled`: A symbol that has an empty/filled interior.
+	(They correspond to Unicode's "white"/"black".)
+- `.tiny`/`.small`/`.medium`/`.big`: A geometric shape with a certain size.
+- `.light`/`.heavy`: A shape with a certain stroke weight.
+- `.dotted`: A shape with a dotted line instead of a full stroke.
+Established concrete modifiers:
+- `.big`: A big (n-ary) version of an operator.
+- `.alt`: An alternative version that is still more or less the same symbol.
+- `.not`: A symbol with a (typically diagonal) line through it.
+- `.o`: A symbol with a circle around it (see [#62](https://github.com/typst/codex/pull/62))
+- `.sq`: A "squarified" version of a symbol (see [#110](https://github.com/typst/codex/pull/110))
+- `.rev`: A horizontally mirrored version of a symbol (see [#108](https://github.com/typst/codex/issues/108))
+- `.inv`: Either vertically mirrored or a 180° rotated version of a symbol (see [#108](https://github.com/typst/codex/issues/108))
+- `.cw`/`.ccw`: Clockwise/Counterclockwise
+Established exceptions:
+- When `.eq` is used in the second sense (stacked below), it only adds a single line and not two.
+	For two lines below, `.equiv` is used.
+- `.not` (see above) does not correspond to the symbol `not` despite having the same name.