Secondary arch freeze exception note added to wiki

[Bruno Wolff III]

The wording I used is:
bugs which constitute infringements of Fedora_Release_Criteria for secondary architectures will be presumed to be valid freeze exception bugs (unless there is a strong argument against, in a particular case) 

I added this to:
https://fedoraproject.org/wiki/QA:SOP_freeze_exception_bug_process#Freeze_exception_bug_principles

I don't know if there is any other place it would be appropriate to add a 
similar note.