[cs][new] first simple code file libe with railwayresult

This is a first file to play with the concept.

Convention of the repo should be 
[language][new | refactor | rewrite | enhanced | breaking | obsolete] commit message

Actually this `file as lib` is tested with [Paket](https://fsprojects.github.io/Paket/)

In the normal way, code files should come with:
- some description : inside the file or an external file ?
- tests : a second file containing the tests for the first one that people could integrate or not (if you're creating `mylib.cs` then you should have `mylib.tests.cs`)