added a contributing guideline

README.md

@@ -8,6 +8,8 @@
 
 This repsitory contains a **collection of minimal Arch wallpapers**, all the themes are made by two colors and look like the above template.
 
+#### If you want to contribute to the repository, check [this file](other/contributing.md)
+
 The themes have been created following the pallete of some famous color schemes, they have been created using two colors:
 
 - The most important color: for the background of the wallpaper.

other/contributing.md

@@ -0,0 +1,26 @@
+## Contributing to the repository
+
+The very first thing you'll have to do is to **fork the repository**, and then **clone to your pc** your copy.
+
+Modify the things you consider, and **commit the changes to your fork**, finally **push the changes** and then **make a pull request** to this repository.
+
+In the following section you can check out more information about adding your own wallpaper:
+
+### Adding a wallpaper
+If you want to add a wallpaper, you'll need [git](git-scm), and an editor (for editing the wallpapers). You can use photoshop, gimp, krita etc.
+
+First, download the following [template](template.png), the template is another wallpaper, but the opacity of it has been reduced to 50%, and I have added some guides in the logo and in the borders.
+
+![Template](template.png)
+
+Open the template in a photo editor, and modify the colors of it. Keep the dimensions. If you need it you can put any of [the wallpapers](../wallpapers/) in another layer, so that you do not have to take care about the guide lines. 
+
+Once you have modified the colors, save the file inside the `wallpapers/` subfolder.
+
+modify the file `other/wallpapers.md`, and **in alphabetical order**, add the name of your wallpaper as:
+
+```markdown
+---
+### name-of-the-wallpaper
+![Name of the wallpaper](../wallpapers/name-of-the-wallpaper.png)
+```