Put together a more thorough CONTRIBUTING guide

[LukeAbby]

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This effectively boils down to the many pitfalls of TypeScript. Common issues seems to be:

• Determining the right type for a plain object. Usually Record<string, unknown>.
• Using InexactPartial appropriately.
• Not writing foo?: T | undefined in function signatures; only for object properties does the distinction between foo?: T and foo?: T | undefined matter.
• Writing effective tests.
• The process of diffing versions.
• Getting started with the helper types.
• etc.

Ideally if lints could be find for these it'd ease the burden into becoming more automated.