Document instructions to add new field to spec

[nvanthao]

Describe the rationale for the suggested feature.

Our contributing guideline does not have section for how to add new field to the schema. This will be useful for new contributor to the project.

Describe the feature

Update the doc with instructions to add new field to the schema. E.g.

# make new changes to Go struct under ./pkg/apis/troubleshoot/v1beta1 OR ./pkg/apis/troubleshoot/v1beta2
# from root directory
make openapischema
make schemas

Describe alternatives you've considered
NA

Additional context
NA