Add content for how to contribute section

[jessicard]

Add roughdraft content for "how to contribute" section.

Fixes https://github.com/hackclub/contribute/issues/1 Fixes https://github.com/hackclub/contribute/issues/9

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[jessicard]

@sampoder @exu3 Merging for quickness sake, but would love your feedback/edits/prs on copy! Also design wise, this isn't feeling super pretty. Any ideas? https://github.com/hackclub/contribute/issues/2