Create CONTRIBUTING.md (#3115)

* Create CONTRIBUTING.md

* Update CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing Guidelines
+
+🎉 **First of all, thanks for taking the time to contribute!** 🎉
+
+## 🤔 How can I contribute?
+
+**1.** Fix issue
+
+**2.** Report bug
+
+**3.** Improve Documentation
+
+
+## Setup 🚀
+You need to set up Textualize to make your contribution. Textual requires Python 3.7 or later (if you have a choice, pick the most recent Python). Textual runs on Linux, macOS, Windows, and probably any OS where Python also runs.
+
+### Installation
+
+**Install Texualize via pip:**
+```bash
+pip install textual
+```
+**Install [Poetry](https://python-poetry.org/)**
+```bash
+curl -sSL https://install.python-poetry.org | python3 -
+```
+**To install all dependencies, run:**
+```bash
+poetry install --all 
+```
+**Make sure everything works fine:**
+```bash
+textual --version
+```
+### Demo
+
+Once you have Textual installed, run the following to get an impression of what it can do:
+
+```bash
+python -m textual
+```
+If Texualize is installed, you should see this:
+<img width="848" alt="demo" src="https://github.com/clairecharles/textual/assets/67120042/62fd53c0-7ad6-4429-8751-5a713180b836">
+
+## Make contribution
+**1.** Fork [this](repo) repository.
+
+**2.** Clone the forked repository.
+
+```bash
+git clone https://github.com/<your-username>/textual.git
+```
+
+**3.** Navigate to the project directory.
+
+```bash
+cd textual
+```
+
+**4.** Create a new [pull request](https://help.github.com/en/github/collaborating-with-issues-and-pull-requests/creating-a-pull-request)
+
+
+### 📣 Pull Requests(PRs)
+
+The process described here should check off these goals:
+
+- [x] Maintain the project's quality.
+- [x] Fix problems that are important to users.
+- [x] The CHANGELOG.md was updated;
+- [x] Your code was formatted with black (make format);
+- [x] All of your code has docstrings in the style of the rest of the codebase;
+- [x]  your code passes all tests (make test); and
+- [x] You added documentation when needed.
+
+### After the PR 🥳
+When you open a PR, your code will be reviewed by one of the Textual maintainers.
+In that review process,
+
+- We will take a look at all of the changes you are making;
+- We might ask for clarifications (why did you do X or Y?);
+- We might ask for more tests/more documentation; and
+- We might ask for some code changes.
+
+The sole purpose of those interactions is to make sure that, in the long run, everyone has the best experience possible with Textual and with the feature you are implementing/fixing.
+
+Don't be discouraged if a reviewer asks for code changes.
+If you go through our history of pull requests, you will see that every single one of the maintainers has had to make changes following a review.
+
+
+
+## 🛑 Important
+
+- Make sure to read the issue instructions carefully. If you are a newbie you should look out for some good first issues because they should be clear enough and sometimes even provide some hints. If something isn't clear, ask for clarification!
+
+- Add docstrings to all of your code (functions, methods, classes, ...). The codebase should have enough examples for you to copy from.
+
+- Write tests for your code.
+
+- If you are fixing a bug, make sure to add regression tests that link to the original issue.
+  
+- If you are implementing a visual element, make sure to add snapshot tests. See below for more details.
+
+  
+### Snapshot Testing
+Snapshot tests ensure that things like widgets look like they are supposed to.
+PR [#1969](https://github.com/Textualize/textual/pull/1969) is a good example of what adding snapshot tests means: it amounts to a change in the file ```tests/snapshot_tests/test_snapshots.py```, that should run an app that you write and compare it against a historic snapshot of what that app should look like.
+
+When you create a new snapshot test, run it with ```pytest -vv tests/snapshot_tests/test_snapshots.py.```
+Because you just created this snapshot test, there is no history to compare against and the test will fail automatically.
+After running the snapshot tests, you should see a link that opens an interface in your browser.
+This interface should show all failing snapshot tests and a side-by-side diff between what the app looked like when it ran VS the historic snapshot.
+
+Make sure your snapshot app looks like it is supposed to and that you didn't break any other snapshot tests.
+If that's the case, you can run ```make test-snapshot-update``` to update the snapshot history with your new snapshot.
+This will write to the file ```tests/snapshot_tests/__snapshots__/test_snapshots.ambr```, that you should NOT modify by hand
+
+
+### 📈Join the community
+
+- 😕 Seems a little overwhelming? Join our community on [Discord](https://discord.gg/uNRPEGCV) to get help.