document keys in man page

st.1

@@ -67,6 +67,31 @@ st executes
 instead of the shell.  If this is used it
 .B must be the last option
 on the command line, as in xterm / rxvt.
+.SH USAGE
+.TP
+.B Ctrl-Print Screen
+toggleprinter()
+.TP
+.B Shift-Print Screen
+printscreen()
+.TP
+.B Print Screen
+printsel()
+.TP
+.B Alt-Shift-Page Up
+increase font size
+.TP
+.B Alt-Shift-Page Down
+decrease font size
+.TP
+.B Alt-Shift-Home
+reset to default font size
+.TP
+.B Shift-Insert
+paste from primary selection
+.TP
+.B Alt-Shift-Insert
+paste from clipboard selection
 .SH CUSTOMIZATION
 .B st
 can be customized by creating a custom config.h and (re)compiling the source