document use of stock OpenCL kernels

[nerdralph]

According to this post, standard C kernels can be used with dcompute:
https://forum.dlang.org/post/lmoiguddkkycfhpuigmb@forum.dlang.org

I don't see any mention of that in the project description or on the main wiki page.

[thewilsonator]

Yeah docs are something that needs polishing. #37 is a attempt to improve that. Feel free to make PRs that you think improve them.