added new section to README #256

[abrilurena]

This Pull request is related to the issue #256: this PR includes updates to the README.md file to provide additional guidance on resource management and cleanup.

Guidance on resource management:

• Added a warning to remind users to take down their app if it's no longer in use to avoid unnecessary costs.

Instructions for resource cleanup:

• Added a new section titled "Clean up" with step-by-step instructions on how to delete all resources created by the sample using the azd down command.

Note: I noticed that the GPT model has already been updated recently.

[nitya]

Hi @abrilurena - thanks for the PR.

1. Wanted to confirm that you are no longer seeing the gpt-35-turbo issues?
2. Can you also check the docs/README.md file and add the same content there? (For other reasons, the two README should ideally be in sync)
3. Also may be worth documenting azd down --purge as an option to use if you also want to also purge deleted resources and recover quota faster.