Add LICENSE file

This repository lacks a LICENSE file, which could cause issues in the further.

On the 2020-12-10 devop meeting it was decided to switch to the MIT license.

We need to ask all former contributors (28 with dups filtered and 35 without) for permission:

$ git shortlog -s -n # please use -e to get the mail adresse 
   626  Florian Pritz
   506  Jelle van der Waa
   352  Sven-Hendrik Haase
   309  Giancarlo Razzolini
   295  Jan Alexander Steffens (heftig)
    89  Bartłomiej Piotrowski
    75  Ira ㋡
    66  Phillip Smith
    53  Frederik Schwan
    31  Jakub Klinkovský
    26  Evangelos Foutras
    22  Christian Rebischke
    19  Kristian Klausen
    13  Levente Polyak
     9  Morten Linderud
     7  Pierre Schmitz
     4  Eli Schwartz
     4  Robin Broda
     3  Santiago Torres
     2  Angel Velasquez
     2  Sébastien Luttringer
     1  Dennis Hoppe
     1  Filipe Laíns
     1  George Rawlinson
     1  Ira ¯\_(ツ)_/¯
     1  Juergen Hoetzel
     1  Justin Kromlinger
     1  nl6720

@dvzrv did this for the archiso project recently (archiso#7 (closed)), so maybe he has some insight.

His mail can be found here: https://lists.archlinux.org/pipermail/arch-releng/2020-May/003964.html and MR here: archiso!5 (merged). We should probably do it the same way (SPDX-License-Identifier + LICENSE file).

Received replies:

• Florian Pritz
• Jelle van der Waa
• Sven-Hendrik Haase
• Giancarlo Razzolini
• Jan Alexander Steffens (heftig)
• Bartłomiej Piotrowski
• Phillip Smith
• Frederik Schwan
• Jakub Klinkovský
• Evangelos Foutras
• Christian Rebischke
• Kristian Klausen
• Levente Polyak
• Morten Linderud
• Pierre Schmitz
• Eli Schwartz
• Robin Broda
• Santiago Torres
• Angel Velasquez
• Sébastien Luttringer
• Dennis Hoppe
• Filipe Laíns
• George Rawlinson
• Ira ¯_(ツ)_/¯
• Juergen Hoetzel
• Justin Kromlinger
• nl6720
• Leonidas Spyropoulos (!239 (merged))
• kpcyrd (!280 (merged))
• Matthias Loibl (!205 (merged))