Add readme for blender plugin

utils/exporters/blender/README.md

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+# Three.js Blender Import/Export
+
+Imports and exports Three.js' ASCII JSON format.
+
+Assumes Blender version 2.60.
+
+## Installation
+
+Copy the io_mesh_threejs folder to the scripts/addons folder. The full path is OS-dependent (see below).
+
+Once that is done, you need to activate the plugin. Open Blender preferences, look for
+Addons, search for `three`, enable the checkbox next to the `Import-Export: three.js format` entry.
+
+Goto Usage.
+
+### Windows
+
+Should look like this:
+
+    C:\Users\USERNAME\AppData\Roaming\Blender Foundation\Blender\2.60\scripts\addons
+
+### OSX
+
+Depends on where blender.app is. Assuming you copied it to your Applications folder:
+
+    /Applications/Blender/blender.app/Contents/MacOS/2.60/scripts/addons
+
+## Usage
+
+Use the regular Import and Export menu within Blender, select `Three.js (js)`.
+