Improvement: Constant Product AMM

[julio4]

Adds more explanations for the Constant Product AMM example.

Details to be Explained:

• approve() the router to spend tokens from the user
• How the swap function works: based on the constant product formula (x * y = k).
• Number of shares / token ratios for add/rm liquidity. Definition of pool

[raizo07]

@julio4 I'll like to be assigned to work on this

[OkoliEvans]

Adds more explanations for the Constant Product AMM example.

Link is broken

[OkoliEvans]

I can take this too if it's okay