Add COLLADA/STL/OBJ examples to README

[EtaLoop]

WHAT

Here is few examples of collada, stl and obj.

Not sure how obj::from_slice works, feel free to correct it.

Closes #60

[taiki-e]

Thanks for the PR!

APIs recommended for most users are Loader::{load,load_from_slice} and its format specific variants (Loader::load_{stl,collada,obj}{,_from_slice}).

https://github.com/openrr/mesh-loader/blob/44ad13656d4b5009f4fb6b172c206d1b9860c18d/examples/kiss3d/src/main.rs#L93-L94

So I would like to use them on top-level examples.

{stl,collada,obj}::from_* are more low-level APIs.

[EtaLoop]

I was referring to the docs.rs modules (stl, collada, obj). It's, indeed, a lot more understandable and convenient.