add gem about interfacing to C/C++

[wilzbach]

trafficstars

From @wilzbach on May 17, 2016 20:31

https://dlang.org/spec/interfaceToC.html

https://dlang.org/spec/cpp_interface.html

https://dlang.org/spec/objc_interface.html

https://dlang.org/spec/abi.html

Copied from original issue: stonemaster/dlang-tour#127

[wilzbach]

From @stonemaster on May 18, 2016 7:16

That's a good idea.