[Docs] Add a note about the plan to switch to Apache License

I propose adding the following note to `docs/content/meta/license.md`:

> *Note: There is a plan to switch to [Apache License, Version 2.0](http://www.apache.org/licenses/LICENSE-2.0), for future Hugo releases.  This change would only apply forward.  Existing versions of Hugo will remain released under the SimPL-2.0 license.  For more information, see [the "Switching to Apache 2 license" thread in the public forum](http://discuss.gohugo.io/t/switching-to-apache-2-license/173).*

Is it appropriate?  Or is it premature?