Added Usage.md file for asset lib build (#59)

addons/tbloader/Usage.md

@@ -0,0 +1,21 @@
+# Usage
+To install TBLoader, you can either install it through [AssetLib](https://godotengine.org/asset-library/asset/1265)
+(search for "TrenchBroom Loader"), or by downloading a [release from Github](https://github.com/codecat/godot-tbloader/releases)
+and extracting it to your project's `addons` folder, so that you have a structure like this:
+```
+project/addons/tbloader/plugin.cfg
+```
+
+You might have to manually enable the plugin from your project settings. In the Godot editor, click
+on Project -> Project Settings, and go to the Addons tab. Check the "Enable" box next to TBLoader.
+
+To build a level's geometry, create a `TBLoader` node in your scene hierarchy. In the properties of
+the node you can select where your `.map` file is located, plus some more useful settings. With the
+node still selected and the 3D view open, you will see a button `Build Meshes` in the toolbar the 3D
+view. Click that button to build the geometry.
+
+# TrenchBroom game config
+The `tb-gameconfig` folder contains a game configuration for this addon. This includes a simple FGD
+which will have some common entities that create Godot nodes. Simply place the files in a folder
+called `Godot` inside the `games` folder of your TrenchBroom installation, so you would have
+`games/Godot/<files>`.