Update README with context usage examples

[lizacodes]

Raised in https://github.com/apollo-server-integrations/apollo-server-integration-aws-lambda/issues/97

With Apollo V4's improved type support for context, it's not currently clear how to use it with this integration without a bit of digging, especially if you are using Typescript and want to make sure you are passing in the correct generics.

With more codebases upgrading to Apollo V4 as a result of V3's end-of-life, it'll be likely more folks will be asking about this.

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[BlenderDude]

Covered in the new V3 release