Add license template in coding guidelines

Merged as part of !2729 (merged)

Context

This MR adds a template for including the license as a header source code files in OCaml (and possibly other languages).

Manually testing the MR

Reviewers and approvers can test this MR by checking whether the new section "License" in the developer/guidelines.rst gives enough information to create and maintain source files, licence-wise.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, the Development Version section of CHANGES.md for everything else).
• Select suitable reviewers using the Reviewers field below.