Add links to elm presentations and workshops possibly targetted at coworkers

[magopian]

Here are a few links that people on slack provided when I asked for resources to present elm to my coworkers, and give them a taste of what elm is :

• @jgrenat workshop: https://github.com/jgrenat/elm-workshop
• @sebbes workshop to code a minesweeper (only in french): https://sebsheep.github.io/elm-workshop/
• @pascal.lemerrer elm initiation (only in french) in prelude to the "payment form workshop": https://speakerdeck.com/pascallemerrer/introduction-pour-latelier-elm-shop
• @pascal.lemerrer initial problem for the "payment form workshop" (participants should fork this and work on it): https://codesandbox.io/s/old-violet-b7teb?file=/src/Main.elm
• @pascal.lemerrer possible solution for the "payment form workshop": https://codesandbox.io/s/competent-monad-wzec3?file=/src/Main.elm:925-968

There are two resources in french that might not be relevant to this repository. The "payment form workshop" is usually animated and solved as a "mob programming" exercise according to Pascal Lemerrer (the author).

Would any of those (and maybe many more in the future) have their place in this repository? It seems it's a recurring question around "I'd like to convince my coworkers to switch to elm".