Clarify GELF 1.1 specification

[joschi]

There are several implementation details missing in the GELF specification, e. g. that using GELF via TCP mandates terminating messages with a null character (\0) or that compression cannot be used (due to the null character being used for message framing).

[mikkolehtisalo]

IMHO size limits for messages and fields should be documented while at it. There are de facto limits after which Graylog will fail parsing, saving, and indexing messages.