include a description of the ontology

[cmungall]

In at least one of these places:

• the README in this repo
• on the OBO page http://obofoundry.org/ontology/pso (click Edit to make a PR)
• the header of the ontology (so it will appear e.g here https://www.ebi.ac.uk/ols/ontologies/pso)

the description should orient the general audience to what kinds of entities are in PSO, what kind of links

at the moment it's not obvious and someone coming here

https://www.ebi.ac.uk/ols/ontologies/pso

could be confused!