Update license header how-to for use of SPDX header

[AndyScherzinger]

Summary

Updates the documentation on updating and creating license headers for making use of SPDX license headers.

Examples can be found at

• https://github.com/nextcloud/profiler
• https://github.com/nextcloud/spreed

With an helper implementation at

• feat(license): Add a script to convert license headers in some files … github_helper#109

TODO

• get it reviewed
• migrate existing headers to SPDX header (optional, can happen afterwards)

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[AndyScherzinger]

looped in the recently involved people regarding use of SPDX headers and also mentioning @skjnldsv since having been part of the discussion of @ShGKme's PR at #38047