add basic usage doc

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+A experimental mouse based 1-bit pixel art editor, written
+in Rust and SDL2. It reads and writes to a homemade image
+format, [OBI](https://git.peppe.rs/graphics/obi).
+
+### Run
+
+Requires SDL2 and SDL2_ttf. 
+
+```
+# creates a new file if it dosen't exist
+cargo run -- <filename>.obi
+```
+
+### Usage
+
+`z/c`: zoom in/out  
+`w/a/s/d`: pan up/left/down/right  
+`Shift-c`: fit iamge to window  
+`q/e`: increase/decrease brush size  
+`1/3`: increase/decrease dither level  
+`x`: flip color  
+`tab`: show/hide grid  
+`i`: invert pixmap (breaks undo for now)  
+`f`: draw line from last point of contact with grid  
+`v`: cycle symmetry options, horizontal, vertical, radial, none  
+`u/r`: undo/redo  
+
+### Lisp REPL
+
+Enter the REPL by pressing `(` on the keyboard. Complete the
+lisp expression with a closing parenthesis and hit enter to
+execute it. The REPL supports basic readline operations.
+
+```
+(define (fac n) (if (<= n 1) 1 (* n (fac (- n 1)))))
+(fac 5)
+```
+
+#### Supported commands:
+
+`(toggle-grid)`: similar to pressing `tab` on the keyboard  
+`(save <filename>)`: saves the current file. optional
+argument to save to a different filename than the one opened
+from.  
+`(quit)`: quit the [editor](editor)
+