Update changelog

CHANGELOG.txt

 nanopb-0.3.8 (2016-03-xx)
  Fix problems with multiple oneofs in same message (#229)
  Zero-valued extension fields were mistakenly ignored by encoder (#242)
- Various fixes related to proto3 encoding (#242, #245, #247)
+ Multiple fixes related to proto3 mode (#242, #245, #247, #249)
  Fix potential unaligned access (#226, #227)
  Fix documentation for protoc --plugin argument (#239)
  Extend inline / fixed length bytes array support (#244)