Remove out-of-date information in README.

[paulmillar]

The README file says:

the releases folder contains the different releases of the PaNET ontology,

the gh-pages branch contains the ontology documentation (produced automatically by a GitHub action).

The first part is plain wrong. It should be updated to document what happens now.

The second part is not wrong, but it should be updated to mention this happens for tagged versions, so the webpage always shows the most recent version.