Add contributing guide to setup dev environment

CONTRIBUTING.md

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+# Setup the environment
+
+1. Clone the repo: `git clone [email protected]:python-lsp/python-lsp-server.git`
+2. Create the virtual environment: `python3 -m virtualenv .venv`
+3. Activate: `source .venv/bin/activate`
+
+Create a helper script to run the server without the need to `pip3 install` it
+on every change and name it `run` or similar:
+
+```py
+#!/home/user/projects/python-lsp-server/.venv/bin/python
+import sys
+
+from pylsp.__main__ import main
+
+sys.exit(main())
+```
+
+## Configure your editor
+
+In Sublime Text 4, open LSP-pylsp settings and change the path to the `pylsp` command:
+
+```json
+{
+    "command": ["/home/user/projects/python-lsp-server/run", "-vvv", "--log-file", "pylsp.log"]
+}
+```
+
+Option `-v` increases verbosity and `--log-file` will write all log messages
+into a file, which can be used for debugging.
+
+Running command `LSP: Troubleshoot server` you should now see the configured command,
+if not, then the configuration doesn't work as expected.
+
+To enabled plugins like `ruff`, you also need to point your editor to the correct
+`PYTHONPATH`:
+
+```json
+{
+    "command": ["/home/user/projects/python-lsp-server/run", "-vvv", "--log-file", "pylsp.log"],
+    "env": {
+        "PYTHONPATH": "/home/user/projects/python-lsp-server/.venv/lib/python3.11/site-packages",
+    },
+}
+```
+
+## Trying out if it works
+
+Go to file `pylsp/python_lsp.py`, function `start_io_lang_server`,
+and on the first line of the function, add some logging:
+
+```py
+log.info("It works!")
+```
+
+Save the file, restart the LSP server and you should see the log line:
+
+```
+2023-10-12 16:46:38,320 CEST - INFO - pylsp._utils - It works!
+```
+
+Now the project is setup in a way you can quickly iterate change you want to add.
+
+# Running tests
+
+1. Install dependencies: `pip3 install .[test]`
+2. Run `pytest`: `pytest -v`