Provide motd command

Summary:
A popular way to connect to a remote shell server is to reattach
to the tmux session. For example, ssh -t eglide.org tmux attach.

In such scenario, motd isn't printed at login time.
To allow to read MOTD in such sc, we provide a convenient command.

Test Plan: motd

Reviewers: Sandlayth, dereckson

Reviewed By: Sandlayth, dereckson

Differential Revision: https://devcentral.nasqueron.org/D817