Document the purpose of sssom-py

[matentzn]

Three core use cases:

1. convert existing formats into SSSOM
2. validate sssom files to encourage high quality FAIR and remixable mappings
3. Export sssom to standard JSON-LD and RDF variants

Secondary use cases:

• Merging mapping sets with very crude reconciliation facilities like excluding lower confidence and negative mappings.
• sorting (canonical outputs)
• etc