Note on getting underlying plain buffer from C code

[svaarala]

Now involves Duktape.Buffer():

• https://github.com/svaarala/duktape/issues/190#issuecomment-155080448

[Type1J]

@svaarala: I tried the first approach that you menchened in #190 (extend duk_push_buffer_object()) before looking at the docs or the Duktape source because there are a few other API calls that intepret their meaning from the type on the top of the stack. I think that that's the most intuitive approach, and as you said, it causes the least problems for future API versions.