Update README

README.md

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 # pytest-subinterpreter
+
+A Pytest extension for running the pytest session inside a [sub interpreter](https://peps.python.org/554). This extension is mainly targeted for Python extension library maintainers that want to verify their existing test suite works inside a sub interpreter.
+
+Requirements:
+
+- Python 3.13.0-alpha.2 or above
+- Pytest 7 or above
+
+## Installation
+
+```console
+pip install pytest-subinterpreter
+```
+
+## Usage
+
+The extension arguments are:
+
+- `--interpreter-per` Defaults to `session` (also the only option at the moment).
+
+```console
+pytest  tests/test_simple.py 
+```