Update changelog

CHANGELOG

+nanopb-0.2.1
+ NOTE: The default callback function signature has changed.
+       If you don't want to update your code, define PB_OLD_CALLBACK_STYLE.
+ 
+ Change the callback function to use void** (issue 69)
+ Add support for defining the nanopb options in a separate file (issue 12)
+ Add support for packed structs in IAR and MSVC (in addition to GCC) (issue 66)
+ Implement error message support for the encoder side (issue 7)
+ Handle unterminated strings when encoding (issue 68)
+ Fix bug with empty strings in repeated string callbacks (issue 73)
+ Fix regression in 0.2.0 with optional callback fields (issue 70)
+ Fix bugs with empty message types (issues 64, 65)
+ Fix some compiler warnings on clang (issue 67)
+ Some portability improvements (issues 60, 62)
+ Various new generator options
+ Improved tests
+
 nanopb-0.2.0
  NOTE: This release requires you to regenerate all .pb.c
        files. Files generated by older versions will not