gabrielecirulli#98 improve contributing guide

Author: gabrielecirulli

CONTRIBUTING.md

@@ -1,2 +1,32 @@
 ## Contributing
-Changes and improvements are more than welcome! Feel free to fork and open a pull request. Please make your changes in a specifically made branch and request to pull on `master`! If you can, please make sure the game fully works before sending the PR, as that will help speed up the process.
+Changes and improvements are more than welcome! Feel free to fork and open a pull request.
+
+Please follow the house rules to have a bigger chance of your contribution being merged.
+
+### House rules
+
+#### How to make changes
+ - To make changes, create a new branch based on `master` (do not create one from `gh-pages` unless strictly necessary) and make them there, then create a Pull Request to master.  
+ `gh-pages` is different from master in that it contains sharing features, analytics and other things that have no direct bearing with the game. `master` is the "pure" version of the game.
+ - If you want to modify the CSS, please edit the SCSS files present in `style/`: `main.scss` and others. Don't edit the `main.css`, because it's supposed to be generated.  
+ In order to compile your SCSS modifications, you need to use the `sass` gem (install it by running `gem install sass` once Ruby is installed). To run SASS, simply use the following command:  
+ `sass --watch style/main.scss`  
+ SASS will automatically recompile your css when changed.
+ - `Rakefile` contains some tasks that help during development. Feel free to add useful tasks if needed.
+ - Please use 2-space indentation when editing the JavaScript. A `.jshintrc` file is present, which will help your code to follow the guidelines if you install and run `jshint`.
+ - Please test your modification thouroughly before submitting your Pull Request.
+
+#### Changes that might not be accepted
+We have to be conservative in regards to the core game. This means that some modifications won't be merged, or will have to be evaluated carefully before doing being merged:
+
+ - Undo/redo features
+ - Save/reload features
+ - Changes to how the tiles look or their contents
+ - Changes to the layout
+ - Changes to the grid size
+
+#### Changes that are welcome
+ - Bug fixes
+ - Compatibility improvements
+ - "Under the hood" enhancements
+ - Small changes that don't have an impact on the core gameplay