Create LICENSE

[lebouquetin]

PR Goal

Add a LICENCE file in order to make licencing more clear. see #304

Implemented solution

Checkpoints

• [x] If relevant, manual tests have been done to ensure the stability of the whole application and that the involved feature works
• [x] Automated tests covering the feature or the fix, have been written, deemed irrelevant (give the reason), or an issue has been created to implement the test (give the link)
• [x] The PR or the original issue contains a short summary of the implemented solution.

[lebouquetin]

@Skylsmoi @Haskaris could you review and merge this?