Add TelnetServer man page

Author: https://github.com/larb0b
Commit: e7c2467231
Pull-request: https://github.com/SerenityOS/serenity/pull/616
Reviewed-by: https://github.com/awesomekling
Reviewed-by: https://github.com/bugaevc

Base/usr/share/man/man8/TelnetServer.md

@@ -0,0 +1,24 @@
+## Name
+
+TelnetServer - Serenity telnet server
+
+## Synopsis
+
+```**sh
+$ TelnetServer [options]
+```
+
+## Description
+
+`TelnetServer` is a basic telnet server for Serenity. By default, it
+runs on port 23 and provides a shell upon connection.
+
+## Options
+* `-p`: Choose different port for TelnetServer to attach to.
+* `-c`: Choose different program for TelnetServer to run on connection.
+
+## Examples
+
+```sh
+$ TelnetServer -p 24 -c /usr/bin/nyancat
+```