Fix manifest to include model card

[osanseviero]

Fixes #135

[osanseviero]

The above was tested by doing

! pip install --upgrade --force-reinstall git+https://github.com/osanseviero/diffusers.git
!ls /usr/local/lib/python3.7/dist-packages/diffusers/utils/

in a Colab, which indeed shows the model_card_template.md file there! 🥳

[anton-l]

Ah, there it is, thank you!

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[anton-l]

We did get the file when installing with git+ before though, so we might still have a problem on the pypi side