docs: add an example binding for copying text to the system clipboard

doc/sample.nanorc.in

@@ -291,6 +291,9 @@
 # bind Sh-M-U "Oc|sed 's/.*/\U&/'
" main
 # bind Sh-M-L "Oc|sed 's/.*/\L&/'
" main
 
+## For copying a marked region to the system clipboard:
+# bind Sh-M-T "|xsel -ib
u" main
+
 ## If you would like nano to have keybindings that are more "usual",
 ## such as ^O for Open, ^F for Find, ^H for Help, and ^Q for Quit,
 ## then uncomment these: