docs: update contributing guide with squash info

[0xmad]

Description

Update contributing guide with commit squashing information

Additional Notes

N/A

Related issue(s)

N/A

Confirmation

• [x] I have read and understand MACI's contributor guidelines and code of conduct.
• [x] I have read and understand MACI's GitHub processes.
• [x] I have read and understand MACI's testing guide.

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Name	Status	Preview	Updated (UTC)
maci-website	✅ Ready (Inspect)	Visit Preview	Jun 24, 2024 8:29pm