Consider giving more visibility to the `build()` method in documentation

[laynor]

Took me a while to discover it, might be nice including it in one of the basic examples

[shepmaster]

Yeah, build is relatively new. It is indirectly demonstrated in https://docs.rs/snafu/0.6.9/snafu/guide/examples/basic/index.html

Out of curiosity, how are you using build? It's needed in some cases, but not in many others.