DOC: add contributing guide - #814

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ℹ️ Scheduling was delayed due to a concurrency limit on community tasks
⚠️ Only [ghcr.io/cirruslabs/macos-runner:sonoma, ghcr.io/cirruslabs/macos-runner:sequoia, ghcr.io/cirruslabs/macos-runner:tahoe] is allowed. Automatically upgraded to ghcr.io/cirruslabs/macos-runner:tahoe.
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