Update README with usage instructions

README

@@ -1,7 +1,8 @@
 SLE
 ===
-Work in progress!
-A simple level editor using the KBLE format.
+A simple project oriented single screen tilebased level editor.
+
+SLE read and write KBLE level files.
 => https://git.sr.ht/~kikoodx/kble
 
 COMPILATION
@@ -19,6 +20,34 @@ $ <SLE binary path> <tileset> <KBLE level to open>
 Example:
 $ build/sle assets/tileset.png sample.kble
 
+USAGE
+=====
+Two windows should appear. A main window, with the level you are
+currently editing, and a secondary one, containing the tileset you
+provided.
+
+This software is controlled using a mice.
+
+Main window - Editing Area
+--------------------------
+Use then:
+* left click: draw with selected tile
+* right click: erase (tile 0)
+
+Secondary window - Tile Picker
+------------------------------
+Left click on the tile you want to select.
+
+Exit and save
+-------------
+Close/kill the main window. The secondary window will close automaticly
+and your modifications to the level file will be writen to disk.
+
+CONFIGURATION
+=============
+Edit include/conf.h to configure the software. Read the comments!
+Don't forget to recompile after you made your modifications.
+
 LICENSE
 =======
 Copyright (C) 2021 KikooDX <kikoodx@paranoici.org>