Add deprecation notice

[mcs07]

Following up on discussion in https://github.com/openbabel/conda-openbabel/issues/7

Some other possible tasks that may be helpful:

• [ ] Disable CI on this conda-openbabel repository
• [x] Add deprecation notice to package summary at https://anaconda.org/openbabel/openbabel
• [ ] Add deprecation notice to this conda-openbabel repository description on github
• [ ] "Archive" this repository in GitHub settings?
• [ ] Add a conda section to Open Babel installation docs
• [ ] Add a short installation summary to Open Babel README?