Add codeowners and readme

CODEOWNERS

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+# These owners will be the default owners for everything in
+# the repo. Unless a later match takes precedence,
+# @primary-owner and @secondary-owner will be requested for
+# review when someone opens a pull request.
+*       @tomjemmett @The-Strategy-Unit/nhp_model_devs

README.md

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+# The New Hospital Programme Demand Model - Databricks implementation
+
+This repository contains code for the Databricks implementation of the NHP model.
+
+The notebooks in this repository can be used to run provider, ICB, and national level runs of the NHP model.
+
+## Explanation of the NHP model Databricks implementation
+
+nhp_model code is a package on GitHub. We tag specific versions of the nhp_model, e.g. v4.2.1.
+
+This package has as its package dependency the tagged version of nhp_model code, available from GitHub.
+So for example, you can specify v4.2.1 or v4.1.0 of nhp_model. This is currently specified in the `pyproject.toml` file.
+
+This repository contains specific Data loading classes for ICB, national and provider level model runs.
+These tell the nhp_model code to load the data for these runs from Databricks.
+
+This repository also has workflow yaml file in the `resources` folder that tell Databricks what to do.
+The workflow is:
+
+1. Look on Azure for params files in a specific folder, then
+1. Run a specific notebook using those params.
+
+nhp_model_databricks uses the same extracted parquet data as the provider level model, just samples from it differently.
+It reads some of the reference tables directly from Databricks.
+
+## How to use the NHP model Databricks implementation
+
+This implementation is still a work in progress.
+We're keeping notes for internal use only in the [nhp_products Wiki](https://github.com/The-Strategy-Unit/nhp_products/wiki/How-to-run-the-ICB-and-national-level-models).