Add LICENSE.txt via license element

[mikkelbu]

Fixes #580

[OsirisTerje]

@mikkelbu Fungerer build -t package ?

[mikkelbu]

@mikkelbu Fungerer build -t package ?

Så vidt jeg har kunnet teste lokalt, så virker package target, og LICENSE.txt er nævnt i metadata. Det ligner også, at det har fungeret på appveyor, https://ci.appveyor.com/project/CharliePoole/nunit3-vs-adapter/builds/26600339#L1809

[OsirisTerje]

Yes, tried locally, it worked :-)

Another thing - wonder why the Azure Pipeline builds have stopped running ? That is, for the pull requests. They work on master

[mikkelbu]

Another thing - wonder why the Azure Pipeline builds have stopped running ? That is, for the pull requests. They work on master

I created nunit/nunit#3338 one week ago and just spent 1 hour without any progress, but I'll examine it some more tonight.

In the NUnit project master was passing jul 28
https://nunit.visualstudio.com/NUnit/_build/results?buildId=1892
but when I triggered a rebuild it failed on aug 3
https://nunit.visualstudio.com/NUnit/_build/results?buildId=1905
and examine the logs it seems that the image has been updated (git-lfs was bumped from 2.7.2 to 2.8.0 and the full change log is here microsoft/azure-pipelines-image-generation@fcd0f07 - I guess).