Add LICENSE file

[valenting]

A license needs to be chosen for this repo.
All contributors need to agree with the chosen license.
A list of contributors is enclosed below:

• Mihai Bivol @mihaibivol
• Mihai Maruseac @mihaimaruseac
• Călin Cruceru @calincru
• Matei Oprea @mateioprea
• @alexandrujuncu

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