Update CONTRIBUTING.md

Spelling fix - removed unnecessary "by"

# Before 

```
# Contributing to Famo.us

Active involvement from the community is essential to help make famous the most
capable and performant front-end JavaScript framework out there. You can help by
reporting bugs, fixing bugs, adding features, contributing new modules and by
by providing feedback.
```

# After
```
# Contributing to Famo.us

Active involvement from the community is essential to help make famous the most
capable and performant front-end JavaScript framework out there. You can help by
reporting bugs, fixing bugs, adding features, contributing new modules and by
providing feedback.
```
Update CONTRIBUTING.md

Fixed broken link 

# Before
```
If you'd like to contribute a fix for a bug you've encountered, first read up on
[how to report a bug][reporting-bugs-and-other-issues] and report it so we are
aware of the issue.
```
# After
```
If you'd like to contribute a fix for a bug you've encountered, first read up on
[how to report a bug](#reporting-bugs-and-other-issues) and report it so we are
aware of the issue.
```
[DOC] CSS classes are set on target div
[TWEAK] Use enum form this._springState
Support function-wrapped transforms in Scene definitions.

Is that the right terminology ("function-wrapped")?

I was trying to do this:

```javascript
    // create the main context
    var mainContext = Engine.createContext();

    // set the perspective (what type of lense?)
    mainContext.setPerspective(1200);

    var initialTime = Date.now();

    // define the main scene.
    var mainScene = new Scene({
        id: "root",
        target: [

            { id: 'background' },

            {
                id: "centerSpin",
                origin: [0.5, 0.5],
                transform: function() {
                    return Transform.rotateY(.001 * (Date.now() - initialTime));
                },
                target: {id: 'logo',}
            }

        ],
    });

    var background = new Surface({
        properties: { backgroundColor: '#25475E'},
    });

        var logo = new ImageSurface({
            size: [400, 400],
            content: '/content/images/logo.png',
            classes: ['backfaceVisibility']
        });

    mainScene.id['logo'].add(logo);
    mainScene.id['background'].add(background);

    mainContext.add(mainScene);
```

but the animation wasn't working. Adding those lines to Scene.js fixed it.
May fixed issue #175
[Fix] Enable chaining for transitionable.halt()