chore: Update readme to clarify initializeFieldsAsUndefined=false.

Author: stephenh

README.markdown

@@ -530,7 +530,7 @@ Generated code will be placed in the Gradle build directory.
 
 - With `--ts_proto_opt=useNumericEnumForJson=true`, the JSON converter (`toJSON`) will encode enum values as int, rather than a string literal.
 
-- With `--ts_proto_opt=initializeFieldsAsUndefined=false`, all optional field initializers will be omitted from the generated base instances.
+- With `--ts_proto_opt=initializeFieldsAsUndefined=false`, all optional field initializers will be omitted from the generated base instances. This can be useful if you're using Next.js and `getServerSideProps`, see [#492](https://github.com/stephenh/ts-proto/issues/492). The default is `true` to ideally get the best v8 performance by avoiding "changing shapes" as fields are incrementally set.
 
 - With `--ts_proto_opt=disableProto2Optionals=true`, all optional fields on proto2 files will not be set to be optional. Please note that this flag is primarily for preserving ts-proto's legacy handling of proto2 files, to avoid breaking changes, and as a result, it is not intended to be used moving forward.