docs(pytest_plugin): Document pytest plugin

docs/index.md

@@ -11,6 +11,7 @@
 quickstart
 cli
 api/index
+pytest-plugin
 
 ```
 

docs/pytest-plugin.md

@@ -0,0 +1,76 @@
+(pytest_plugin)=
+
+# `pytest` plugin
+
+Download and reuse UNIHAN.zip on the fly in [pytest].
+
+```{module} unihan_etl.pytest_plugin
+
+```
+
+[pytest]: https://docs.pytest.org/
+
+## Usage
+
+Install `unihan-etl` via the python package manager of your choosing, e.g.
+
+```console
+$ pip install unihan-etl
+```
+
+The pytest plugin will automatically be detected via pytest, and the fixtures will be added.
+
+## Fixtures
+
+`pytest-unihan-etl` works through providing {ref}`pytest fixtures <pytest:fixtures-api>` - so read up on
+those!
+
+The plugin's fixtures guarantee downloading, and then reusing UNIHAN.zip every
+test without needing to redownload.
+
+(recommended-fixtures)=
+
+## Recommended fixtures
+
+These are fixtures are automatically used when the plugin is enabled and `pytest` is ran.
+
+- Creating temporary, test directories for:
+  - `/home/` ({func}`home_path`)
+  - `/home/${user}` ({func}`user_path`)
+- Setting your home directory
+  - Patch `$HOME` to point to {func}`user_path` ({func}`set_home`)
+## Bootstrapping pytest in your `conftest.py`
+
+The most common scenario is you will want to configure the above fixtures with `autouse`.
+
+_Why doesn't the plugin automatically add them?_ It's part of being a decent pytest plugin and
+python package: explicitness.
+
+(set_home)=
+
+### Setting a temporary home directory
+
+```python
+import pytest
+
+@pytest.fixture(autouse=True)
+def setup(
+    set_home: None,
+):
+    pass
+```
+
+## See examples
+
+View unihan-etl's own [tests/](https://github.com/cihai/unihan-etl/tree/master/tests)
+
+## API reference
+
+```{eval-rst}
+.. automodule:: unihan_etl.pytest_plugin
+    :members:
+    :inherited-members:
+    :private-members:
+    :show-inheritance:
+    :member-order: bysource
+```