[license] Add custom MIT license for fmt

[license] Add Zlib license for tinyxml2

[license] Add BSD-2-Clause for doubleexp integration

[license] add CC-BY-4.0 license for code of conduct

[license] reference copyright holder in AUTHORS.md

* Remove statement duplicated from README.md
* Move contributors to a separate file AUTHORS.md
* Mention AUTHORS.md file in the README.md