Add collada examples

[EtaLoop]

Hello, Could you please add simple Usage in README.md.

Something like that :

let cube = r##"<?xml version="1.0" encoding="utf-8"?>
        <COLLADA xmlns="http://www.collada.org/2005/11/COLLADASchema" version="1.4.1" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
        <asset>
            <contributor>
            <author>Blender User</author>
            <authoring_tool>Blender 2.79.0 commit date:2018-03-22, commit time:14:10, hash:f4dc9f9d68b</authoring_tool>
            </contributor>
            <created>2018-11-19T22:54:36</created>
            <modified>2018-11-19T22:54:36</modified>
            <unit name="meter" meter="1"/>
            <up_axis>Z_UP</up_axis>
        </asset>
        <library_images/>
        <library_effects>
            <effect id="Material-effect">
            <profile_COMMON>
                <technique sid="common">
                <phong>
                <ambient>
                <color sid="ambient">0 0 0 1</color>
                </ambient>
                <diffuse>
                <color sid="diffuse">0.64 0.64 0.64 1</color>
                </diffuse>
                <shininess>
                <float sid="shininess">50</float>
                </shininess>
                <index_of_refraction>
                <float sid="index_of_refraction">1</float>
                </index_of_refraction>
                </phong>
                </technique>
            </profile_COMMON>
            </effect>
        </library_effects>
        <library_materials>
            <material id="Material-material" name="Material">
            <instance_effect url="#Material-effect"/>
            </material>
        </library_materials>
        <library_geometries>
            <geometry id="Cube_001-mesh" name="Cube.001">
            <mesh>
                <source id="Cube_001-mesh-positions">
                <float_array id="Cube_001-mesh-positions-array" count="24">-0.5 -0.5 -0.5 -0.5 -0.5 0.5 -0.5 0.5 -0.5 -0.5 0.5 0.5 0.5 -0.5 -0.5 0.5 -0.5 0.5 0.5 0.5 -0.5 0.5 0.5 0.5</float_array>
                <technique_common>
                    <accessor source="#Cube_001-mesh-positions-array" count="8" stride="3">
                    <param name="X" type="float"/>
                    <param name="Y" type="float"/>
                    <param name="Z" type="float"/>
                    </accessor>
                </technique_common>
                </source>
                <source id="Cube_001-mesh-normals">
                <float_array id="Cube_001-mesh-normals-array" count="18">-1 0 0 0 1 0 1 0 0 0 -1 0 0 0 -1 0 0 1</float_array>
                <technique_common>
                    <accessor source="#Cube_001-mesh-normals-array" count="6" stride="3">
                    <param name="X" type="float"/>
                    <param name="Y" type="float"/>
                    <param name="Z" type="float"/>
                    </accessor>
                </technique_common>
                </source>
                <vertices id="Cube_001-mesh-vertices">
                <input semantic="POSITION" source="#Cube_001-mesh-positions"/>
                </vertices>
                <triangles material="Material-material" count="12">
                <input semantic="VERTEX" source="#Cube_001-mesh-vertices" offset="0"/>
                <input semantic="NORMAL" source="#Cube_001-mesh-normals" offset="1"/>
                <p>1 0 2 0 0 0 3 1 6 1 2 1 7 2 4 2 6 2 5 3 0 3 4 3 6 4 0 4 2 4 3 5 5 5 7 5 1 0 3 0 2 0 3 1 7 1 6 1 7 2 5 2 4 2 5 3 1 3 0 3 6 4 4 4 0 4 3 5 1 5 5 5</p>
                </triangles>
            </mesh>
            </geometry>
        </library_geometries>
        <library_controllers/>
        <library_visual_scenes>
            <visual_scene id="Scene" name="Scene">
            <node id="Cube" name="Cube" type="NODE">
                <matrix sid="transform">1 0 0 0 0 1 0 0 0 0 1 0 0 0 0 1</matrix>
                <instance_geometry url="#Cube_001-mesh" name="Cube">
                <bind_material>
                    <technique_common>
                    <instance_material symbol="Material-material" target="#Material-material"/>
                    </technique_common>
                </bind_material>
                </instance_geometry>
            </node>
            </visual_scene>
        </library_visual_scenes>
        <scene>
            <instance_visual_scene url="#Scene"/>
        </scene>
        </COLLADA>"##;

    let doc = mesh_loader::collada::from_str(&cube);

    if let Ok(doc) = doc {
        for mesh in &doc.meshes {
            assert_eq!(mesh.name, "Cube_001-mesh");
            println!("Mesh name : {}", mesh.name);
            println!("Vertices : {:?}\n", mesh.vertices);
            println!("Normals : {:?}\n", mesh.normals);
            println!("Textcoords : {:?}\n", mesh.texcoords);
            println!("Faces : {:?}\n", mesh.faces);
            println!("Color : {:?}\n", mesh.colors);
        }
        for material in &doc.materials {
            assert_eq!(material.opacity, None);
            assert_eq!(material.shininess, Some(50.0));
            assert_eq!(material.index_of_refraction, Some(1.0));
            assert_eq!(material.color.diffuse, Some([0.64, 0.64, 0.64, 1.0]));
            assert_eq!(material.color.ambient, Some([0.0, 0.0, 0.0, 1.0]));
            assert_eq!(material.texture.diffuse, None);
            assert_eq!(material.texture.ambient, None);
        }
    }