[gimp-web] content: add a section on how to contribute to tutorials.

[Jehan <jehanp src gnome org>]

commit 85ce3d47021b7ab35397b59d58700172ab7e73a3
Author: Jehan <jehan girinstud io>
Date:   Wed Nov 10 20:51:29 2021 +0100

    content: add a section on how to contribute to tutorials.
    
    Adding this info there could help increase awareness on the possibility
    of contributing fixes, changes and new tutorials oneselves.

 content/tutorials/index.md | 79 +++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 75 insertions(+), 4 deletions(-)
---
diff --git a/content/tutorials/index.md b/content/tutorials/index.md
index e890200e..887e7e73 100644
--- a/content/tutorials/index.md
+++ b/content/tutorials/index.md
@@ -4,6 +4,12 @@ Modified: 2015-07-29T14:40:43-05:00
 Authors: Pat David
 Summary: The tutorials page.
 
+🛈 As everything created by the GIMP project, these tutorials are fully
+community-written. Therefore if you find any tutorial incomplete,
+or if it contains errors or outdated information, the best thing is to
+contribute a fix. Moreover we warmly welcome new tutorials. [See below
+for the full contribution
+procedure](#contributing-new-tutorials-or-fixes).🛈
 
 ## Beginner
 
@@ -84,14 +90,79 @@ Using GIMP Python to automate a workflow.
 [Automatic Creation of XCF from JPG][]  
 Import XCF images a directory at a time.
 
----
+## Contributing new tutorials or fixes
+
+You don't have to be a developer to participate to the GIMP project.
+Some team members help with design, community management, bug triaging…
+and some help with documentation and tutorials!
+
+The only thing we ask of contributors is to use *Libre* licenses, which
+allow anyone to share, redistribute and modify the tutorials. It enables
+others to fix tutorials, even years later, even if the original author
+is not around anymore. This is the same rules as for our code, making
+GIMP a Free Software.
+
+<span id="recommended-licenses">**Recommended licenses**</a> are:
+
+* [Free Art 1.3](https://artlibre.org/licence/lal/en/)
+* Creative Commons [Attribution-ShareAlike 4.0 
International](https://creativecommons.org/licenses/by-sa/4.0/)
+* Creative Commons [Attribution 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/)
+* [CC0 1.0 Universal](https://creativecommons.org/publicdomain/zero/1.0/).
+
+So you want to contribute? The process is simple, here is an example for
+the hypothetical case where you found an error in an existing tutorial.
+In such case of editing existing text, your change will be automatically
+considered to be in the same license as chosen by the first author:
+
+1. All tutorial contents can be found on our [website's
+   repository](https://gitlab.gnome.org/Infrastructure/gimp-web/-/tree/testing/content/tutorials)
+   (*Gitlab*). Feel free to browse to understand the structure: for each
+   tutorial, you have a directory with inside the file `index.md` which
+   contains the text of the tutorial. Many other files can accompany the
+   tutorial inside the directory, such as images, screenshots and
+   various assets.
+2. Once you found a tutorial, which you want to edit, click the
+   `index.md` link, and you will see a blue button saying "Edit". Click
+   it (obviously you need to have created a login first and be
+   connected).
+3. *Gitlab* will propose you to "fork" the project because of missing
+   permissions.
+4. Once this is done, fix what needs to be using the
+   [Markdown](https://daringfireball.net/projects/markdown/syntax)
+   syntax, as for existing text.
+5. At the bottom of the edit page, notice the "Commit message" field.
+   Fill in relevant information explaining what you did. Click "Commit
+   changes".
+6. A "New merge request" page will appear. Click "Create merge request".
+7. It's done! Now all you have to do is wait for one of the core members
+   to review and validate your changes.
+
+If you just need to change an image (e.g. because it shows an outdated
+version of GIMP), you may select the image in *Gitlab* and click the
+"Replace" button. It will propose you to upload a new version and to
+write a "Commit message" to explain the change. Please write down the
+[desired *Libre* license](#recommended-licenses) for the image in the
+commit message. Then hit "Replace file".
+
+For more complicated changes, or if you want to propose new tutorials,
+it might be necessary to understand at least the basics of the *git*
+source versionning system we use.
+Alternatively, you may simply write your tutorial in a new text file on
+your machine (following *Markdown* syntax), then simply [open a
+report](https://gitlab.gnome.org/Infrastructure/gimp-web/-/issues/new)
+and upload all the files (the text file as well as images and other
+resources) in this report. Do not forget to tell us the license you
+chose from [the list given above](#recommended-licenses).
 
-The list of legacy tutorials can be found found here:
+---
 
+<small>
+A list of legacy tutorials can be found here:
 [All (Legacy) Tutorials List]({filename}list-all.md)
 
-<small>
-Bear in mind that this list is being provided for legacy reasons only.  
+Bear in mind that this list is being provided for legacy reasons only.
+They were originally made for very old hence outdated versions of GIMP
+and most of them do no use Libre licenses allowing editing.
 </small>
 
 [Basic GIMP Perl]: {filename}Basic_Perl/index.md