Update README to describe the maven repo to add

[Corrob]

I had some trouble figuring this out, and figured I would save others some trouble if they had the same issue.

[mk-5]

hello :) 👍 good point. There is some WIKI page with information about the repository https://github.com/mk-5/gdx-fireapp/wiki/Maven but it will be much better to put that inside the main README.md.

One remark here: it should be available through the jcenter() repository, did you test it? Something like this:

allprojects {
    ...
    repositories {
        ...
        jcenter()
    }
}

It should work, if that's fine for you please change full URL to jcenter() and I will merge it

[Corrob]

Yes, that works! I only had mavenCentral() and google() before.