Add .gitattributes for Solidity syntax highlighting on GitHub

[Noel Maersk]

An example can be found in the OpenZeppelin repo:

https://github.com/OpenZeppelin/openzeppelin-solidity/blob/master/.gitattributes

[ledgerwatch]

Thank you, I have done it now, maybe I need to wait for GitHub to realise

[ledgerwatch]

Works now, thank you so much!