Update docs with variables.yml and stage

[raymondbutcher]

This line of code:

serverless-builder/serverless/service/__init__.py

Line 60 in 4e0783a

self.custom = YamlOrderedDict(vars="${file(./variables.yml):${sls:stage}}", **(custom or {}))

Makes it necessary to have a variables.yml file and pass in --stage when running serverless. You'll get errors if they are missing.

It would be good to mention these requirements in the Getting started docs.

Alternatively/additionally it could check for the existence of that file and skip it when it doesn't exist.