Add example for cImportPaths

[jmh530]

It was suggested on the forum (here) that it would be good to have an example of how to use cImportPaths with dub and a C project.

[kinke]

https://github.com/dlang/dub/tree/master/test/issue2698-cimportpaths-broken-with-dmd-ldc?

[jmh530]

@kinke Nice. Then can be easily adapted for example folder.