[Custom runtimes] Add readme for eoapi.dev deployment

The `eoapi.dev` endpoints are deployed using the custom runtimes hosted in this repo. We use custom deployment code (should be similar to the one found in eoapi-template) and we should document (locally) how to update the stack

cc @emileten

Read the existing custom runtime deployment code in this repository and compare it with the approach in eoapi-template. Document locally how the eoapi.dev stack is updated, including the deployment steps and any relevant differences; done means a newcomer can follow the README to update the stack.

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