add py.typed for mypy checks to package

CHANGELOG.md

@@ -1,3 +1,7 @@
+## [Unreleased]
+- Include the PEP 561 `py.typed` marker so mypy recognizes the installed package's type annotations.
+- describe metadata models in JSON terms in their JSON schemas
+
 ## [0.1.0] - 2026-10-28
 - Expand documentation with installation instructions, a metadata model reference, shared examples, and clearer API descriptions
 - Add validated, immutable metadata models with helpers to read, attach, update, and serialize them. Pandas operations share attached models instead of repeatedly copying metadata. Attached models are stored under attrs["hd_metadata"]; use attrs_to_json before JSON export.

README.md

@@ -21,6 +21,9 @@ Since version >0.13.10 the hetida designer runtime comes with an installed versi
 The plotting helpers (`hdhelpers.plotting` and their names in `hdhelpers`) need plotly, which is the optional extra
 `plotting`: `pip install "hdhelpers[plotting]"`. Everything else, e.g. the metadata models, works without plotly.
 
+The package ships inline type annotations and a `py.typed` marker (PEP 561), so type checkers such as
+mypy can use its type information after installation.
+
 An example is given in [GitHub page](https://hetida.github.io/hdhelpers/first_steps.html#how-to-get-metadata-with-hdhelpers) of how to use functionalities of hdhelpers inside a hetida designer component. Furthermore the base component [Single Time Series Plot](https://github.com/hetida/hetida-designer/blob/release/runtime/transformations/components/visualization) uses hdhelpers to demonstrate the usage (since version >0.13.10).
 
 ## Developing for hdhelpers