Licence mention added to all files

[TristanAllaire]

Thank you for your contribution to Taquito.

Before submitting this PR, please make sure:

• Your code builds cleanly without any errors or warnings
• You have run the linter against the changes
• You have added unit tests (if relevant/appropriate)
• You have added integration tests (if relevant/appropriate)
• All public methods or types have TypeDoc coverage with a complete description, and ideally an @example
• You have added or updated corresponding documentation
• If relevant, you have written a first draft summary describing the change for inclusion in Release Notes.

In this PR, please also make sure:

• You have linked this PR to the issue by putting closes #TICKETNUMBER in the description box (when applicable)
• You have added a concise description on your changes

Release Note Draft Snippet

If relevant, please write a summary of your change that will be suitable for
inclusion in the Release Notes for the next Taquito release.