Add elixir badge

Signed-off-by: John Lago <[email protected]>

Author: Lagoja

examples/development/elixir/elixir_hello/README.md

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 Basic Elixir project using Mix in Devbox.
 
+[![Open In Devspace](https://www.jetify.com/img/devbox/open-in-devspace.svg)](https://www.jetify.com/devbox/templates/elixir)
+
 ## Configuration
 
 This project configures Hex and Mix to install packages + dependencies in local project directories. You can modify where these packages are installed by changing the variables in `conf/set-env.sh`