Add License

[Eeems]

Since there is no license, it's not clear what others are allowed to do with your code.

[Eeems]

Upon further review it looks like you've specified that this is under the MIT license in the source file:
https://github.com/simonschllng/rm-sync/blob/master/rM-sync.sh#L6

The full license text should still be included so that Github can properly identify that this is under MIT.