Add --callback and --callback-help to man page

speedtest-cli.1

@@ -58,6 +58,16 @@ URL of the Speedtest Mini server
 Source IP address to bind to
 .RE
 
+\fB\-\-callback CALLBACK\fR
+.RS
+Script to run at certain stages of operation
+.RE
+
+\fB\-\-callback\-help\fR
+.RS
+Show help about --callback
+.RE
+
 \fB\-\-version\fR
 .RS
 Show the version number and exit