feat: lake: add readme to package templates

[alok]

Messaged @tydeu about adding a README.md to new lake projects. I decided to add it with the help of GPT.

[tydeu]

@alok Thanks! Looks good! There is a type error that needs fixing, though.

[leanprover-community-mathlib4-bot]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 799923d145960c6f2515b510111df2c170b34ca3 --onto 91244b2dd9d223006227648659203373f5a46b0b. (2024-05-14 18:54:51)

[tydeu]

@alok Any reason why you still have this marked as draft?

[alok]

Thanks for reminder, updated

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[tydeu]

Thanks!