docs: revisit `README.rst`

### Problem

As discussed in the #165, the content in `README.rst` like
```
recommended that you consult online resources and become somewhat
familiar before using PDFmorph.

PDFmorph can be run with Python 3.10 or higher. It makes use of several third party
```
is a little outdated

### Proposed solution

Revisit `README.rst` and bring it up to date.