Add `LICENSE` and `requirements.txt` + update `MANIFEST.in`

[sugatoray]

• [x] add LICENSE file: #62
• [x] include requirements.txt in PyPI source: #64
• [x] update the MANIFEST.in
• [x] add PEP 561 compliance: just add an emplty file py.typed under modelcards folder.

Closes #62, #64

---

Note:

I am packaging skops for conda-forge. This library is a dependency of skops and hence I am adding this one. The packaging of modelcards has a few issues. This PR should fix them and the next released version of modelcards should make conda-forge packaging rather straight-forward. For now, am using a temporary workaround to package the libraries on conda-forge.

• https://github.com/conda-forge/staged-recipes/pull/20031

[sugatoray]

cc: @nateraw