Add a guide for contributing.

CONTRIBUTING.md

+
+
+## Git
+
+### The easy way
+
+Use Gitlab's WebIDE.
+You won't be able to 100% accurately preview the result until you commit and the servers are updated.
+
+### The powerful way
+
+Run the website locally and preview your changes instantly.
+Head over to the [sources of the server instance](https://m3p2.ljbac.com/m3p2/grav-website) to learn how to set it up.
+
+
+## File Structure
+
+Each page is a directory.
+
+
+### File names
+
+We define as *slug-case* what some call *kebab-case*.
+
+- lowercase
+- alphanumeric
+- joined with `-`
+- `a-z`, no diacritics
+- starts with a letter (probably best)
+
+
+#### Directories
+
+`[<NN>.]<slug>`
+
+The numbers prefix can be ignored when building links, its purpose is to order sibling pages.
+
+Set the english slug ; localized slugs can be set in frontmatters like so:
+
+```yaml
+title: Joyeux Noël!
+slug: joyeux-noel
+```
+
+
+#### Markdown files
+
+`<template>[.<lang>].md`
+
+The *name of the file* will define the *template of the page*, that is how the content will appear.
+
+> For example, the template `columns` will display the children of the page as columns.
+
+The available templates are :
+- `default`
+- `portal`
+- `topics`
+- `topic`
+- `columns`
+- `textbook`
+- `cheatsheet`
+- `annex`
+
+Right now there are not many differences between the various templates but there may be later.
+
+
+
+#### Other files
+
+- Use **slug-case** with the occasional snake between slugs.
+- Be expressive.  This is an exercise in aphorisms. `fig-a_rays-through-convex-lens.png`
+- use a ISO 639-1 code as suffix like an extension like so: `textbook.fr.md`.
+
+
+
+### Markdown Cheatsheet
+
+```
+# Header <h1>
+## Header <h2>
+…
+###### Header <h6>
+
+*emphasis*
+**strong**
+
+Go [there](https://…)
+[Current chapter](.)
+[Parent chapter](..)
+[Sibling chapter](../another-chapter)
+[Child chapter](chapter)
+
+![Description of the image](local-image.png)
+
+! blue notice
+!! green notice
+!!! orange notice
+!!!! red notice
+!!!!
+!!!! still the same red notice
+```
+
+> More at Gitlab's [Markdown documentation](https://docs.gitlab.com/ee/user/markdown.html).
+
+
+### LateX
+
+You can use LateX in the courses :
+
+    $`E = m \cdot c^2`$
+
+yields $`E = m \cdot c^2`$
+
+> You can omit the _backticks_ (`) but the preview in gitlab won't work.
+
+
+### Mermaid
+
+You can display flowcharts in the courses :
+
+    ``` mermaid
+    graph LR
+        subgraph Course
+            File[File]
+        end
+    
+        Student((Student))
+    
+        Student --> |reads| File
+    ```
+
+yields
+
+``` mermaid
+graph LR
+  subgraph Course
+    File[File]
+  end
+
+  Student((Student))
+
+  Student --> |reads| File
+```
+
+> `LR` is for Left to Right. You could use `TB` for Top to Bottom.
+
+GANTT diagrams and sequence diagrams are also available.
+Read on [mermaid's documentation](https://mermaidjs.github.io/) for more information.
+
+
+### Table demo
+
+| A | B |   | D |   |
+|---|---|---|---|---|
+|   |   |   |   |   |
+|   |   | X |   |   |
+|   |   | X |   | _test_  |

README.md

@@ -2,56 +2,12 @@
 
 A flat-file database of courses in multiple languages.
 
-## Documentation
+## Browse
 
-- Read the extensive [Markdown documentation](https://docs.gitlab.com/ee/user/markdown.html).
+The courses are [available here](https://courses.m3p2.ljbac.com).
 
------
+## Contribute
 
-### LateX demo
+Anyone can contribute to the courses, in a wikipedia manner.
 
-You can use LateX in the courses :
-
+Read [CONTRIBUTING](CONTRIBUTING.md) for more information.
\ No newline at end of file
-    $`E = m \cdot c^2`$
-
-yields $`E = m \cdot c^2`$
-
-> You can omit the _backticks_ (`) but the preview in gitlab won't work.
-
-### Mermaid demo
-
-You can use graphs in the courses :
-
-
-    ``` mermaid
-    graph LR
-    subgraph Course
-        File[File]
-    end
-
-    Student((Student))
-
-    Student --> |reads| File
-    ```
-
-yields
-
-``` mermaid
-graph LR
-  subgraph Course
-    File[File]
-  end
-
-  Student((Student))
-
-  Student --> |reads| File
-```
-
-
-### Table demo
-
-| A | B |   | D |   |
-|---|---|---|---|---|
-|   |   |   |   |   |
-|   |   | X |   |   |
-|   |   | X |   | _test_  |