Document the .options file usage.

Also add note about the 'packed' message option being incompatible
with CPUs that do not support unaligned access.

Update issue 12
Status: FixedInGit

Update issue 77
Status: FixedInGit

generator/nanopb.proto

@@ -33,6 +33,8 @@ message NanoPBOptions {
   optional bool long_names = 4 [default = true];
   
   // Add 'packed' attribute to generated structs.
+  // Note: this cannot be used on CPUs that break on unaligned
+  // accesses to variables.
   optional bool packed_struct = 5 [default = false];
 }