Documentation for glib dependency

[jack-ullery]

In an exhaustion fueled craze, I added a glib dependency last night when creating the aa-replace binary. Now I need to add documentation for this dependency in the README. I should also consider if there is another way to code aa-replace that does not add a dependency.

Creating a glib dependency shouldn't be a huge issue, because it is also present in the greater AppAnvil project.