Clarify when/how Docker is needed in docs about fluidframework

[curtisman]

Docker is not strictly necessary for building packages/libraries. For client side code, that is probably enough to target our internal routerlicious deployment. But in order to test server code locally or for those that doesn't have access to routerlicious deployment, docker how one can run the test server locally.

[curtisman]

Probably worth putting that in README.md somewhere.

[markfields]

@curtisman can you take this or delegate to someone else? (I can't assign it right now so just pinging you instead)

[curtisman]

https://github.com/microsoft/FluidFramework/wiki/Client and https://github.com/microsoft/FluidFramework/wiki/Server spells it out now