Clarify optionality discovery mechanisms in section 8.4

[mlagally]

The current text in the discovery chapter needs to be augmented with a statement,
that implementers are not obliged to adopt the two exploration mechanisms defined in the discovery spec:

We want to encourage but not make the following mandatory:

Two exploration mechanisms are provided, supporting both peer-to-peer discovery and directory services. Peer-to-peer discovery allows a Thing to provide its own WoT Thing Description directly. Directory services scalably maintain searchable databases of WoT Thing Descriptions. Both lightweight syntactic query mechanisms and more powerful semantic query mechanisms are supported by directory services.