Create Functions Framework Spec

[benjaminhuo]

OpenFunction already have implemented the functions-framework-go and functions-framework-nodejs, the functions-framework-python, functions-framework-dotnet and functions-framework-java is also under developing.

We need a spec to guide the development of each language's functions framework.

Here is a draft of such a Spec https://hackmd.io/@xcEqMy2IQx-DQu_nUuPNXw/HkXqDW6P5
We need to create an English version of this and add more content to it based on each language's functions framework's experiences.

The author of each language's functions framework should contribute to this doc

@tpiperatgod @webup @wanjunlei @kehuili @geffzhang