Add edts recipe

https://github.com/tjarvstrand/edts
https://travis-ci.org/tjarvstrand/edts/builds/22904550

I am the maintainer of this package.

The Erlang Development Tool Suite (EDTS) is a package of useful development tools for working with the Erlang programming language in Emacs. It bundles a number of useful external packages, together with specialized Erlang plugins for them, and its own features to create a complete and efficient development environment that is easy to set up.

The reason for not using the master branch is that the melpa-package branch contains installation instructions and enables a check whether EDTS was installed as a package or not. I don't want to put these things into the master branch before the package is actually available in melpa. As soon as the package is available, I will merge the melpa-package and submit a new pull request with a recipe pointing to master.