REUSE compliance

[nirs]

We are very close to be compliant with the REUSE specification:

% reuse lint
# MISSING LICENSES

'Apache-2.0' found in:
* .gitignore
* CONTRIBUTING.md
* Makefile
* README.md
* cmd/commands/init.go
* cmd/commands/root.go
* cmd/commands/test.go
* cmd/commands/validate.go
* cmd/main.go
* docs/user-interface.md
* examples/odf.go
* pkg/config/config.go
* pkg/console/console.go
* pkg/test/clean.go
* pkg/test/run.go
* pkg/validate/clusters.go


# MISSING COPYRIGHT AND LICENSING INFORMATION

The following files have no copyright and licensing information:
* go.mod
* go.sum
* pkg/config/sample.yaml

The following files have no licensing information:
* DCO

# SUMMARY

* Bad licenses: 0
* Deprecated licenses: 0
* Licenses without file extension: 0
* Missing licenses: Apache-2.0
* Unused licenses: 0
* Used licenses: Apache-2.0
* Read errors: 0
* Files with copyright information: 17 / 20
* Files with license information: 16 / 20

Unfortunately, your project is not compliant with version 3.3 of the REUSE Specification :-(


# RECOMMENDATIONS

* Fix missing licenses: For at least one of the license identifiers provided by
  the 'SPDX-License-Identifier' tags, there is no corresponding license text
  file in the 'LICENSES' directory. For SPDX license identifiers, you can simply
  run 'reuse download --all' to get any missing ones. For custom licenses
  (starting with 'LicenseRef-'), you need to add these files yourself.
* Fix missing copyright/licensing information: For one or more files, the tool
  cannot find copyright and/or licensing information. You typically do this by
  adding 'SPDX-FileCopyrightText' and 'SPDX-License-Identifier' tags to each
  file. The tutorial explains additional ways to do this:
  <https://reuse.software/tutorial/>

Tasks:

• Add missing SPDX headers either as text or as .license sidecar file
• Add reuse lint CI job