Edits to how-to-contribute-to-appflowy draft

[egp415]

• Added spacing for clarity
• Removed unecessary HTML formatting
• Ensured consistency of AppFlowy branding (i.e. case)
• Made sure heading hierarchy was consistent
• Fixed use of unordered lists
• Enforced stylistic rule for links (i.e. make links contain relevant text)
• Edited for clarity

[CLAassistant]

All committers have signed the CLA.