Create LICENSE.md

Add license.

Closes modelica#2292

Note that:
- I could find neither an official text-version of this, nor an official markdown version.
- The official instructions state that you should link as above (but in html); and does not say that you should copy the text.

LICENSE.md

@@ -0,0 +1,6 @@
+Copyright (C) 1998-2020 Modelica Association Project "Modelica Language"
+              All rights reserved.
+
+![Creative Commons License](https://i.creativecommons.org/l/by-sa/4.0/88x31.png)
+
+The Modelica Specification is licensed under a [Creative Commons Attribution-ShareAlike 4.0 International License](https://creativecommons.org/licenses/by-sa/4.0/).