Protocol: Deprecate "string value" fields for future versions of the spec about venue-taxonomy

[mowasfi7]

+1

[christiancollins11]

+1 per comments on other threads

[bruno8g]

+1

[jayshao]

This issues was created as part of discussion against #52 around renaming categories

[jayshao]

Notes from #49

• some implementors are using string fields as part of their implementation
• we don't want to suddenly cutoff something when it's being used
• what happens when labels change? renames / localization?
• should OpenRTB be machine-to-machine
• could announce, but target removal towards the end of the year? (1.3? 1.4?)
• alternative: change the semantics of the spec to pass human-readable, but not guarantee machine parse-ability and compatibility

Proposal: Deprecate string values with removal in 1.4 (+8. -1)