doc(): add contributing guidelines

CONTRIBUTING.md

@@ -0,0 +1,19 @@
+# Contributing
+
+## Getting started
+
+- Git clone the repo in a Hugo site's theme directory : `git clone https://github.com/aubm/hugo-code-editor-theme.git /path/to/site/themes/code-editor`
+- Go in to the site's directory : `cd /path/to/site`
+- Serve the website with Hugo : `hugo server`
+- Open a new terminal and go in to the theme's directory : `cd /path/to/site/themes/code-editor`
+- Install dependencies with npm : `npm i`
+- Build static files : `npm run build`
+
+## Code
+
+Please do not edit the `theme.css`, `theme.min.css`, `theme.js` or `theme.min.js` files directly. Prefer working on source 
+files and use the `npm run watch` command to automatically rebuild on edit.
+
+## Commit message guidelines
+
+Please try to keep a clean history and use the [official Angular guidelines](https://github.com/angular/angular/blob/master/CONTRIBUTING.md#-commit-message-guidelines) for commit messages.

README.md

@@ -4,4 +4,8 @@ A theme for [Hugo](https://gohugo.io/), inspired by Atom editor.
 
 ![Theme preview](static/img/theme-preview.png?raw=true "Code Editor Theme for Hugo")
 
-Changelog [here](https://github.com/aubm/hugo-code-editor-theme/blob/master/CHANGELOG.md) :)
+Changelog [here](https://github.com/aubm/hugo-code-editor-theme/blob/master/CHANGELOG.md) :)
+
+# Contributing
+
+Contributions are welcome. Please refer to the [contributions guidelines](https://github.com/aubm/hugo-code-editor-theme/blob/master/CONTRIBUTING.md) for more information.