added contribution guide

Author: amirreza8002

CONTRIBUTING.md

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+Hi!
+thanks for considering contribution to this project.
+
+this is a community project and maintained by people, so we appreciate any help.
+
+if you want to add a new future, please open an [issue](https://github.com/amirreza8002/django-async-extensions/issues) first.
+
+Requirements:
+1. python >= 3.10
+2. poetry >= 2.0
+
+
+contribution steps:
+
+1. fork the project from the [repository](https://github.com/amirreza8002/django-async-extensions)
+2. clone the forked repo (you can find it in your own repositories)
+3. go to the cloned project's directory (probably `cd django-async-extensions` will do it for you)
+
+4. install dependencies:
+    this project uses [poetry](https://python-poetry.org/) to manage dependencies, so you want to install poetry first if you haven't.
+    
+    in the project directory (where `pyproject.toml` exists) run `poetry sync --with dev` to get the packages.
+    if you are writing documentations, run `poetry sync --with doc`
+    if you want to use ipython so you can `await` in the shell, run `poetry sync --with ipython`
+    if you need al of these, run `poetry sync --all-groups`
+
+5. activate the virtual env:
+    to activate poetry follow one of the ways explained [here](https://python-poetry.org/docs/managing-environments/#activating-the-environment) depending on your OS and shell.
+
+6. write the code/document you want
+
+7. testing:
+    tests are written using pytest.
+    to run the test suits simply run `python tests/runtest.py`
+    currently these flags are avilable:
+    * --quite
+    * -v={0,2,3}
+    * --failfast
+    * --keepdb
+
+8. documentation:
+    docs are written using [mkdocs](https://www.mkdocs.org/)
+    you can run `mkdocs serve` so you can see the changes you make as you develop.
+
+9. git:
+    we expect a clean git history, you can use the many sources available online to see how you can keep a clean git history.
+
+10. sending the codes:
+    first push your changes to your forked repo (using `git push origin main` or the like)
+    then go to the repo and click on the button saying `Contribute`
+    then open a pull request, we'll check the code and either accept or add notes to it.