Add readme to badyaml usecase

Justify the significance of [badyaml](https://github.com/apache/incubator-openwhisk-wskdeploy/tree/master/tests/usecases/badyaml) usecase by adding README.md. If its not used/applicable anymore, delete badyaml use case.

Start in tests/usecases/badyaml and inspect repository references to determine whether this use case is still used or applicable. If it remains relevant, document its purpose and significance in README.md; otherwise remove the badyaml use case. Confirm the chosen outcome by checking that references and use-case organization remain consistent.

Written by the indexing model from the issue text.