Add tutorial about non-system libraries

[pvdrz]

fixes #1299

[emilio]

I think we probably want to generally point just at how linking external libs (and point out that bindgen doesn't build or link them). But a minimal example with something like:

rust-bindgen/bindgen-integration/build.rs

Lines 151 to 155 in 0142c0a

	cc::Build::new()
	.cpp(true)
	.file("cpp/Test.cc")
	.include("include")
	.compile("libtest.a");

which is all that's needed to build + link a simple c/c++ file, would probably be ok.

[emilio]

(CI is failing, due to the broken comment above)

[emilio]

Alright, that's fair.