[Introduction] Revise the description about the constraints of the Koordinator PriorityClass and QoS class

[saintube]

The current introduction about Priority and QoS in Koordinator does not clarify the constraints between them.
However, some combinations of PriorityClassName and QoS are exactly forbidden, which sometimes confuse the users and developers.

Here we expect to add a section to the Koordinator website, stating the constraints between the Priority and QoS.