Add a demo of mermaid to the README.

README.md

 # Courses
 
-A flat-file database of courses in multiple languages.
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+A flat-file database of courses in multiple languages.
+
+
+### 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
+```
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