DOC: document how to run tests

Author: lucascolley

README.md

@@ -1,2 +1,25 @@
 # xsf
 Special function implementations
+
+## Tests
+
+To run the tests:
+- [clone this repository](https://docs.github.com/en/repositories/creating-and-managing-repositories/cloning-a-repository)
+- `cd xsf`
+- [install Pixi](https://pixi.sh/latest/#installation)
+- `pixi run tests`
+
+For subsequent test runs, you can skip re-cloning [`xsref`](https://github.com/scipy/xsref) with:
+
+```shell
+pixi run --skip-deps tests
+```
+
+You can trigger a rebuild inbetween test runs with:
+
+```shell
+pixi run build-tests
+```
+
+> [!NOTE]  
+> This has currently only been tested on Linux.