Add example of the process for updating a schema to the README.

README.md

@@ -1 +1,64 @@
-New collection of schemas for Mk2
+New collection of schemas for Mk2
+
+
+## Modifying a schema
+
+The below is a walkthrough of the steps required to make a change to an existing schema, by way of
+an example. Other changes will require slightly different steps.
+
+In this example, we wish to modify the type of a field (`accessibility.formatAndStandards.conformsTo`) 
+in the HDRUK 2.2.1 schema.
+
+### Modify schema files
+
+The schema definition in `hdr_schemata/models/HDRUK/2.2.1/schema.json` is 
+generated from the contents of `hdr_schemata/models/HDRUK/v2_2_1`, building 
+upon the contents of previous versions. `v2_1_2 is the "base model" all
+current schemas are derived from.
+
+We create a new `FormatAndStandards.py` in the
+`v2_2_1` directory, with the boilerplate taken from `v2_1_2/FormatAndStandards.py`. Into this file we import 
+```python
+from hdr_schemata.models.HDRUK.v2_2_0 import (
+    FormatAndStandards as BaseFormatAndStandards,
+)
+```
+Note that this is from the previous version, which itself imports from `v2_1_2`. Ensure that the `v2_1_2/__init__.py` correctly exports the class.
+
+Overload the class, using the imported class as base:
+```python
+class FormatAndStandards(BaseFormatAndStandards):
+    conformsTo: Optional[List[StandardisedDataModels]] = Field(
+        ..., **an.vocabularyEncodingScheme.__dict__
+    )
+```
+
+Now import this file into `v2_2_1/Accessibility.py`, and overload the field there with the new type:
+
+```python
+class Accessibility(BaseAccessibility):
+    access: Access = Field(..., description=an.description, title=an.title)
+
+    formatAndStandards: Optional[FormatAndStandards] = Field(
+        None,
+        title=an.formatAndStandards.title,
+        description=an.formatAndStandards.description,
+    )
+```
+
+The changes to the schema are now ready to process.
+
+### Create schema json
+
+Run `python create_json_schema.py`. This will modify the contents of `hdr_schemata/models/HDRUK/2.2.1/schema.json` as appropriate. You may need to install the repo as a local package:
+```bash
+pip install -e .
+```
+
+### Update docs
+
+Finally, update the Markdown docs with:
+```bash
+python hdr_schemata/utils/create_markdown.py
+```
+to generate the Markdown in `docs/`. After merging, this will be available at `https://hdruk.github.io/schemata-2/`.