Add --preview option

This feature adds a preview rectangle which shows the current hovered
color at the left bottom corner in a 50x50 pixel area.

.gitignore

@@ -0,0 +1 @@
+colorpicker

Makefile

@@ -1,7 +1,10 @@
+.PHONY: clean
 
-colorpicker : main.c
-	cc -o colorpicker main.c `pkg-config --libs --cflags gtk+-2.0 gdk-2.0 x11`
+CFLAGS := $(shell pkg-config --libs --cflags \
+	gtk+-2.0 gdk-2.0 x11 xcomposite xfixes)
 
-.PHONY : clean
-clean :
+colorpicker: main.c
+	cc -o colorpicker main.c $(CFLAGS)
+
+clean:
 	rm -f colorpicker

README.md

@@ -1,12 +1,52 @@
 # colorpicker
 
-A small tool for X11 that writes the color value on your screen at the cursor position to stdout, in RGB.
+A small tool for X11 that writes the color value on your screen at the cursor
+position to stdout, in RGB.
 
 ### Usage
+
 Left click to print the pixel color, any other mouse click to quit the program.
 
+#### One Shot
+
+In order to just select one pixel run colorpicker with the `--one-shot` option.
+The program will then quit after the first click.
+
+#### Output Format
+
+By default the program prints out the color in RGA format and in hexadecimal.
+Here is an example:
+```
+R:  44, G: 190, B:  78 | Hex: #2CBE4E
+```
+
+With the help of the `--short` option you can force colorpicker to just
+print out the hexadecimal value. Then you just get: `#2CBE4E`.
+
+#### Color Preview
+
+colorpicker allows you to show a preview of the color the currently hovered
+pixel in a reactangle. This reactangle is located at the bottom left of your
+screen.  This comes in handy in combination with the one shot option so you
+don't pick blindly a color. Just add the `--preview` option for this feature.
+
+#### Examples
+
+```bash
+# Pick a color and put the hexadecimal value in your clipboard
+$ colorpicker --short --one-shot | xsel -b
+
+# Pick a color with preview and put the hexadecimal value in your clipboard
+$ colorpicker --short --one-shot --preview | xsel -b
+```
+
 ### Dependencies
-GTK/GDK 2.0.
+
+* GTK/GDK 2.0
+* X11
+* Xcomposite
+* Xfixes
 
 ### License
+
 MIT