Add license to Github repo

[Bultako]

This PR adds LICENSE.rst at the root level so to make possible Zenodo syncing with Github, now failing with the following error.

{
"errors": "The license ID you have selected is not present in our system. For the available licenses please check in the following URL https://developers.zenodo.org/#licenses"
} 

https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/licensing-a-repository

[adonath]

I guess the 2nd question is whether the license file we have is compatible with any of the valid licenses...

[codecov]

Codecov Report

Merging #3656 (cc101cb) into master (d5d2e12) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3656   +/-   ##
=======================================
  Coverage   93.76%   93.76%           
=======================================
  Files         162      162           
  Lines       19507    19507           
=======================================
  Hits        18290    18290           
  Misses       1217     1217           

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[Bultako]

I have installed and run in my laptop licensee which is the tool used by Github to detect licenses from files, as it is said here

It seems that the license is well detected from the LICENSE.rst file.

~/git/gammapy/gammapy$ licensee detect .
License:        BSD-3-Clause
Matched files:  LICENSE.rst
LICENSE.rst:
  Content hash:  da0ec2854bfc396385cfc49501ed09e1a19b015b
  Attribution:   Copyright (c) 2014, Gammapy developers
  Confidence:    99.17%
  Matcher:       Licensee::Matchers::Dice
  License:       BSD-3-Clause
  Closest non-matching licenses:
    BSD-3-Clause similarity:        99.17%
    BSD-4-Clause similarity:        85.51%
    BSD-3-Clause-Clear similarity:  84.40%

[adonath]

Thanks @Bultako, no further comments from my side.