Add limitation note about subclass type override (#35)

README.md

@@ -65,6 +65,7 @@ Until then, minor and patch releases may contain breaking changes.
 ### Known Current Limitations
 
 - **Inlined imports**: Imported Pkl types are inlined into the output TypeScript file. For example, if `foo.pkl` has an import like `import "bar.pkl"`, and you run `pkl-gen-typescript foo.pkl`, the resulting `foo.pkl.ts` file will include all types defined in `foo.pkl` _as well as_ all types defined in `bar.pkl`. This means that the resulting TypeScript generated files (in a multi-file codegen) will match the set of input root files, not the file structure of the source Pkl files. This behaviour may create unintended name conflicts; these can be resolved using the `@typescript.Name { value = "..." }` annotation. It may also cause duplication (eg. if the same shared Pkl library file is imported in two schemas); TypeScript's structural typing (where equivalent type shapes can be used interchangeably) should mean that any duplicate types can be safely used as each other.
+- **Subclass type overrides**: Pkl class definitions are generated as TypeScript interfaces in code generation. However, TypeScript interfaces, when using `extends` to extend a parent type, do not support overriding the type of a property from the parent. Even though this _is_ supported in Pkl subclassing, it is not currently possible to generate TypeScript for Pkl classes that override parent property types.
 - **Regex deserialisation**: Pkl's `Regex` type will be decoded as a `pklTypescript.Regex` object, which contains a `.pattern` property. Pkl uses Java's regular expression syntax, which may not always be perfectly compatible with JavaScript's regular expression syntax. If you want to use your Pkl `Regex` as a JavaScript `RegExp`, and you are confident that the expression will behave the same way in JavaScript as in Pkl, you can instantiate a new `RegExp` using the `pklTypescript.Regex.pattern` property, eg. `const myConfigRegexp = new RegExp(myConfig.someRegex.pattern)`.
 - **IntSeq deserialisation**: Pkl's `IntSeq` type is intended to be used internally within a Pkl program to create a range loop. It is unlikely to be useful as a property type in JavaScript, and is therefore decoded into a custom `pklTypescript.IntSeq` type with signature `{ start: number; end: number: step: number }` - it is _not_ decoded into an array containing the ranged values. If you have a use-case to use `IntSeq` as an array of ranged values in a TypeScript program, please file a GitHub Issue.
 - **Duration and DataSize APIs**: Pkl has a rich API for many of its custom types, but two of note (that are not common in standard libraries of other languages) are `Duration` and `DataSize`, which include convenience APIs for eg. converting between units or summing values. These types are decoded into `pklTypescript.DataSize`/`pklTypescript.Duration` types (each of which have a `value` and `unit` property), and do not yet have the convenience APIs from Pkl.