Add information on how to contribute

CONTRIBUTING.md

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+## How to contribute to the CTSM@UiO documentation
+
+- step 1: Fork this repository as shown in the figure below.
+
+<img src="img/fork_ctsm-docs.png" alt="Fork CTSM documentation repository">
+
+- step 2: Go online to [CTSM documentation](https://ctsm-docs.readthedocs.io/en/latest/) and whenever you would like to update the documentation, click on "Edit on GitHub".
+
+<img src="img/edit_on_github.png" alt="Edit documentation online">
+
+- step 3: Then click on the "pen" (see image below) and write your text ([reStructuredText](http://docutils.sourceforge.net/docs/user/rst/quickref.html)) 
+
+
+<img src="img/edit_in_your_fork.png" alt="Edit the file in your fork">
+
+- step 4: Save your changes in your forked repository and create a pull request.
+
+
+<img src="img/propose_changes.png" alt="Propose your changes">
+
+
+If you do not like to update the documentation online and prefer to use your favorite editor locally on your machine/laptop, you can skip step-3 and 4 and then clone your forked repository to edit the files locally. Once pushed to your forked github repository, you can create a pull request and propose your changes.