(github) add contributing file

Author: antoine92190

.github/CONTRIBUTING.md

@@ -0,0 +1,13 @@
+# Contributing to vue-advanced-chat
+
+:+1::tada: First off, thanks for taking the time to contribute! :tada::+1:
+
+- [Fork](https://github.com/antoine92190/vue-advanced-chat) the vue-advanced-chat repository
+- Clone the repository using `git clone https://github.com/YOUR_USERNAME/vue-advanced-chat`
+- Go inside your cloned repository and run `npm install`
+- Go inside the `demo` folder and run `npm install`
+- If you want to test using Firebase, you can follow the steps [here](https://github.com/antoine92190/vue-advanced-chat#using-with-firestore)
+- You can use the code in `demo/src/ChatContainer.vue` or remove it entirely and write your own code to test the chat
+- Run `npm run serve` inside the `demo` folder to open the app
+- You need to keep `import ChatWindow from './../../src/ChatWindow'` to use the chat code locally and see your modifications live
+- When you are done with your changes, you can create a pull request and make sure you filled in the template correctly