List all entity type updates in the spec

[pietercolpaert]

From the DCAT-AP Feeds call of 10th of January with @matthiaspalmer:

• Add a requirement: one entity per graph activity
• We should list all DCAT-AP entities and how to add them as a graph (todo @matthiaspalmer) in a section 1.1
• Add a note about order: make sure to use a timestamp path, and make sure no entities get added that refer to not yet existing entities (to be discussed: exception for entities that refer to one another?)