LICENSE not included

Currently the license is not being included in the `conda-forge` version of this package. Newer `conda-forge` packages require this. This results in **all future updates of the feedstock failing** because the license file can not be found. For an example see https://github.com/conda-forge/multipledispatch-feedstock/pull/10.  It can probably be fixed by creating a `MANIFEST.in` in this repository with the following in it: ``` include LICENSE.txt ``` Afterwards the [feedstock](https://github.com/conda-forge/multipledispatch-feedstock) can be adapted to use this newly included license file. I am happy to take over the fixing of the feedstock and already have a local version of the changes (probably) needed in this repository, but am not authorized to push a new branch. What do you think? Including the license file in the distribution seems sensible to me and you probably also want the feedstock to remain maintainable.