Describe schema tests and test wrapper script in CONTRIBUTING.md

[ralfhandl]

• how to run tests
• how to add tests
• how to validate Overlays with the validate.mjs script here

[jeremyfiel]

hi @ralfhandl

is this sufficient to close this issue? https://github.com/OAI/Overlay-Specification/blob/main/schemas/v1.0/readme.md

[ralfhandl]

CONTRIBUTING.md would be a better place for this information, the README.md is somewhat hidden.