=BG= add easy instructions for upgrading from `broccoli-writer` to `broccoli-caching-writer`
Merge pull request #10 from bguiz/patch-1

=BG= add easy instructions for upgrading from `broccoli-writer` to `broccoli-caching-writer`
Only check cache for included or excluded files:

For example, here’s a hypothetical configuration for broccoli-compass so that changing other files (like `*.js` or `*.html`) in a tree does not cause a rebuild:

```javascript
var outputTree = compileCompass(inputTree, {
  filterFromCache: {
    include: [
      '**/*.{scss,sass}'    // only base the input tree’s hash on *.scss and *.sass files
    ]
  }
});
```

Note, this does _not_ affect what files make it to the output tree at all, rather it only makes it easier for subclasses to only rebuild when file types they care about change.