Add multi phoenix umbrella and umbrella with submodules setup

[Aridjar]

While deploying an umbrella app with two phoenix apps and git submodules, I encountered multiple problems.

This file explain how I corrected them and guide to working around these problems.

Note: The file may not be the correctly placed depending of your documentation rules. It may also contains error, as English isn't my main language. Please let me know if you want any changes, I'll be happy to apply them.

Note2: It could be interesting to put the first paragraph of the umbrella app config from the deploy.html#umbrella to this file.

[Aridjar]

And I forgot to mention the need of the master_proxy, though it is already mentioned in the how-do-i-deploy-an-umbrella-app part