Add config.yml linting
Linting the config is very useful, especially when the document is sometimes wrong.
e.g: "Defines rules for allowing/blocking execution of some branches if Workflows are not configured and you are using 2.0 (not 2.1) config. If you are using Workflows, job-level branches will be ignored and must be configured in the Workflows section of your
config.yml file. If you are using 2.1 config, you will need to add a workflow in order to use filtering. See the workflows section for details. The job-level
branch key takes a map:" as in the documentation, but when I used 2.0, it still return error.