doc: update book on metadata + Python (#754)

Author: molpopgen

book/src/SUMMARY.md

@@ -19,7 +19,7 @@
 * [Metadata](./metadata.md)
   - [Defining metadata types in rust](./metadata_derive.md)
   - [Metadata and tables](./metadata_tables.md)
-  - [Metadata schema](./metadata_schema.md)
+  - [Metadata processing with Python](./metadata_python.md)
   - [Advanced topics](./metadata_advanced.md)
 
 * [Error handling](./error_handling.md)

book/src/metadata_python.md

@@ -0,0 +1,31 @@
+# Metadata processing with Python
+
+## `JSON` metadata
+
+If your metadata are generated in `JSON` format via `serde` (see [here](metadata_derive.md)), then the metadata are simple to access from Python.
+The code repository for `tskit-rust` contains examples in the `python/` subdirectory.
+
+You may work with `JSON` metadata with or without a metadata schema (see [here](https://tskit.dev/tskit/docs/stable/metadata.html)).
+A schema is useful for data validation but there is an unfortunate inefficiency if your input to Python is a tree sequence rather than a table collection.
+You will have to copy the tables, add the metadata schema, and regenerate a tree sequence.
+See the examples mentioned above.
+
+## Other formats
+
+The `tskit-python` API only supports `JSON` and Python's `struct` data formats.
+It is useful to use a format other than `JSON` in order to minimize storage requirements.
+However, doing so will require that you provide a method to covert the data into a valid Python object.
+
+An easy way to provide conversion methods is to use [pyo3](https://pyo3.rs) to create a small Python module to deserialize your metadata into Python objects.
+The `tskit-rust` code repository contains an example of this in the `python/` subdirectory.
+The module is shown in its entirety below:
+
+```rust, noplaygound, ignore
+{{#include ../../python/tskit_glue/src/lib.rs}}
+```
+
+Using it in Python is just a matter of importing the module:
+
+```python
+{{#include ../../python/test_bincode_metadata.py}}
+```