Add note about using non-d kernels

[thewilsonator]

41

[thewilsonator]

@nerdralph Does this cover what you think is needed in terms of docs?

[nerdralph]

Not really. "you need to ensure is that the (mangled) name and signature of the kernels D declaration match with its definition in the other language and you can use it as is it were a D kernel." That means I have to learn how D does name mangling. An example OpenCL kernel invocation would probably clear that up.