update readme.md to include ESM example

[blazing234]

update readme.md to include ESM example

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[blazing234]

@kenneth-rosario Would this type of PR be able to be approved? I want to edit the cloud docs in general to include more ESM examples

[matthewrobertson]

We have a guide in docs that described how to use ESM: https://github.com/GoogleCloudPlatform/functions-framework-nodejs/blob/main/docs/esm.md

I don't think we want to pull this into the main README as we want to keep in minimal.