Add LICENSE and CONTRIBUTING.md
Adds a license and CONTRIBUTING.md
License check for dependencies in https://gitlab.com/gitlab-org/analytics-section/product-analytics/gl-application-sdk-browser/-/issues/47#note_2033843369
Confirmation that Contribution.md is fine in https://gitlab.com/gitlab-com/legal-and-compliance/-/issues/2261#note_2037728657