Add source code packaging for GPL compliance

[tzl-index545]

Obviously, Hydro is licensed under the GPL, so providing a “download source code” feature is essential. It is recommended to add a build-time mechanism to automatically package the source code for download.

[undefined-moe]

You can always retrive it from GitHub -- as linked in the footer -- and trust me it's not that hard for someone that really needs the source code.

[tzl-index545]

I mean that some may modify the code but forget to publish it. Add a notice or something can significantly reduce such cases.