DOCS: update README

README.md

@@ -5,17 +5,56 @@
 
 -----
 
+This package provides a Hatch plugin for configuring "variants" of dependencies according to an environment variable.
+This can be used e.g. to change the package dependencies for conda-forge vs PyPI builds.
+
 ## Table of Contents
 
 - [Installation](#installation)
 - [License](#license)
 
-## Installation
+## Global dependency
+
+Ensure `hatch-deps-selector` is defined within the `build-system.requires` field in your `pyproject.toml` file.
 
-```console
-pip install hatch-deps-selector
+```toml
+[build-system]
+requires = ["hatchling", "hatch-deps-selector"]
+build-backend = "hatchling.build"
 ```
 
+## Build plugin
+
+The [build plugin](https://hatch.pypa.io/latest/plugins/build-hook/reference/) name is `selector`.
+
+- ***pyproject.toml***
+
+    ```toml
+    [tool.hatch.build.hooks.selector]
+    env-var = <ENV-VAR-NAME>
+
+    [tool.hatch.build.hooks.selector.variants.<VARIANT>]
+    dependencies = ["numpy"]
+    ```
+
+- ***hatch.toml***
+
+    ```toml
+    [build.hooks.selector.variants.PYPI]
+    dependencies = ["numpy"]
+    ```
+
+By default, set `HATCH_SELECTOR_VARIANT=<VARIANT>` to select the dependencies from `<VARIANT>` as additional project dependencies.
+This might be used to only pull in certain dependencies when building for PyPI vs conda-forge.
+
+
+### Build plugin options
+
+| Option     | Type   | Default                  | Description                                            |
+|------------|--------|--------------------------|--------------------------------------------------------|
+| `env-var`  | `str`  | `HATCH_SELECTOR_VARIANT` | Name of environment variable to control built variant. |
+| `variants` | `dict` | `{}`                     | Table of variant-tables with `dependencies` field.     |
+
 ## License
 
 `hatch-deps-selector` is distributed under the terms of the [MIT](https://spdx.org/licenses/MIT.html) license.