document icon source debug

doc/icons.md

@@ -0,0 +1,28 @@
+Why alttab doesn't show the icon or shows an unexpected icon?
+=============================================================
+
+First, read the description of the `-s` option in `man alttab`.  
+If the issue is still unclear:
+
+Build alttab with `ICON_DEBUG=1` and start it with `-v` option:
+
+```shell
+./configure CFLAGS="-DICON_DEBUG=1"
+make
+src/alttab -v (...other options)
+```
+
+Make the switcher appear, and then find the following fragment in the alttab
+stderr output:
+
+```
+got N windows
+0: 1c0000e (lvl 0, icon 65011716 (32x32)): window name 0
+   <source of icon>
+1: 3c00003 (lvl 0, icon 65011737 (32x32)): window name 1
+   <source of icon>
+...
+```
+
+The source of the icon should help you figure out what's happening.  
+If still in trouble, [file a bug report](https://github.com/sagb/alttab/issues).