Document the key bindings

Document them in the README, the man page, and the help message.

Author: edgar-bonet

README.md

@@ -225,6 +225,21 @@ ttyplot also supports *counter* style metrics, calculating *rate* by measured ti
  
 
 
+## key bindings
+
+
+when reading data from a pipe, ttyplot accepts the following commands typed at the terminal:
+
+```
+q   quit
+r   toggle "rate mode" on and off
+```
+
+these commands do not work if the standard input is a terminal: in this case quit with Ctrl-C.
+
+ 
+ 
+
 
 ## frequently questioned answers
 

ttyplot.1

@@ -83,6 +83,13 @@ Print the current version and exit.
 .It Fl h
 Print this help message and exit.
 .El
+.Sh KEY BINDINGS
+When reading data from a pipe, ttyplot accepts the following commands typed at the terminal:
+.Bl -tag -width Ds
+.It Ic q
+Quit.
+.It Ic r
+Toggle "rate mode" on and off.
 .Sh EXAMPLES
 CPU usage from
 .Xr vmstat 8

ttyplot.c

@@ -91,7 +91,9 @@ static void usage(void) {
             "  -t title of the plot\n"
             "  -u unit displayed beside vertical bar\n"
             "  -v print the current version and exit\n"
-            "  -h print this help message and exit\n\n");
+            "  -h print this help message and exit\n"
+            "\n"
+            "Interactive commands: q = quit, r = toggle rate mode\n");
 }
 
 static void version(void) {