Document available features.

We are not currently keeping track of features as they are added, and as such there is no easy way for a use to test these new features without manually searching the source.

This PR adds `FEATURES.md` as a place for contributors to indicate what each feature flag does and where we can find out more information about it. As new features are added we can require that this file be kept up to date.

I chose to not utilize the `CHANGELOG` for this as it is modified by the build tools, and these features do not apply to a specific version of Ember until they have been selected and included into a stable release.

When a given feature is merged into stable and 'un-flagged' we can remove it from this list.