Updated doc

README.md

@@ -69,6 +69,7 @@ Inside image view mode, the following key mappings are available:
     <,>          Rotate image (counter-)clockwise by 90 degrees
     W            Resize window to fit image
     a            Toggle anti-aliasing
+    A            Toggle visibility of alpha-channel, i.e. transparency
     r            Reload image
     Return       Switch to thumbnail mode
 

sxiv.1

@@ -146,6 +146,9 @@ Toggle fullscreen mode.
 .B a
 Toggle anti-aliasing.
 .TP
+.B A
+Toggle visibility of alpha-channel, i.e. image transparency.
+.TP
 .B r
 Reload image.
 .SH MOUSE COMMANDS