Use standard REUSE formatting for licenses

## Feature Request

### Description of Problem:
This repository currently has multiple licenses applied (for good reasons!). It is not clear which license is associated with which content in the repository.

### Potential Solutions:
The standard REUSE formatting and approach for licenses supports tight alignment of licenses with individual files (https://reuse.software/tutorial) through a developing standard. There is also a helper tool that can be set up as a GitHub Action (see https://github.com/marketplace/actions/reuse-compliance-check) to ensure compliance at commit.

Highlighted by @vmbrasseur (https://github.com/finos/InnerSource/pull/39#issuecomment-963318776). Needs to be discussed by @mcleo-d, @amaltson and @copiesofcopies against the wider FINOS governance and boilerplates.