Add documentation about Mermaid

[esoubiran-aneo]

Here the content

Mermaid is a JavaScript based diagramming and charting tool. Recently, github added support to render mermaid code directly into the web browser. For example the code

```mermaid
stateDiagram 
    direction LR
    [*] --> State1
    State1 --> [*]: A happy transition
    State1 --> State2

    %% This is how you declare composite states
    state State2 {
        [*] --> task_ready
        task_ready --> CheckWhy: no
        task_ready --> Continue: yes
    }
    State2 --> State2
    note left of State2 : A happy note

    %% This is how you make forks
    state fork_state <<fork>>
    State2 --> fork_state
    fork_state --> fork1
    fork_state --> fork2

    %% and how to merge them
    state join_state <<join>>
    fork1 --> join_state
    fork2 --> join_state

    %% And this how specify concurrency
    state State3 {
        [*]-->async1
        --
        [*]-->async2
        --
        [*]-->async3
    }

    join_state --> State3
    State3 --> [*]
```

will be rendered as

stateDiagram direction LR [*] --> State1 State1 --> [*]: A happy transition State1 --> State2 %% This is how you declare composite states state State2 { [*] --> task_ready task_ready --> CheckWhy: no task_ready --> Continue: yes } State2 --> State2 note left of State2 : A happy note %% This is how you make forks state fork_state <> State2 --> fork_state fork_state --> fork1 fork_state --> fork2 %% and how to merge them state join_state <> fork1 --> join_state fork2 --> join_state %% And this how specify concurrency state State3 { [*]-->async1 -- [*]-->async2 -- [*]-->async3 } join_state --> State3 State3 --> [*]

You can use this live editor to produce your charts and then just put them into a markdown file in your repository. Have fun :)