docs: add morph badge (#2666)
Adds the Morph badge to the README. Supersedes #2663. The badge renders
correctly on the branch, as seen below.
<img width="924" alt="image"
src="https://github.com/Unstructured-IO/unstructured/assets/1635179/3dce2e6f-ce9d-452c-a0a7-4077ec7d66ce">
