write: add page (#1577)

pages/linux/write.md

@@ -0,0 +1,16 @@
+# write
+
+> Write a message on the terminal of a specified logged in user (ctrl-C to stop writing messages).
+> Use the `who` command to find out all terminal_ids of all active users active on the system.
+
+- Send a message to a given user on a given terminal id:
+
+`write {{username}} {{terminal_id}}`
+
+- Send message to "testuser" on terminal "/dev/tty/5":
+
+`write {{testuser}} {{tty/5}}`
+
+- Send message to "jhondoe" on pseudo terminal "/dev/pts/5":
+
+`write {{jhondoe}} {{pts/5}}`