Add FIT IoT-LAB as Leshan adopters

Merged Christopher Guindon requested to merge github/fork/sbernard31/iotlab_adopter into master

Created by: sbernard31

I ask to IoT-LAB by email and get an answer from Saint-Marcel Frederic(INRIA) :

It's with great pleasure that you can mention IoT-LAB project as a Leshan adopter.

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