Added small note that mosh needs to be installed on both the client and server

[eminence]

It occasionally comes up in #mosh that people are unsure if mosh needs to be installed on the server. I've added a small bit of text which should hopefully clear this up.

Here is a screenshot of the rendered change:

[edouard-lopez]

Add some bold on both the client and server so it more salient/visible.

[keithw]

Thanks! Deployed to http://mosh.mit.edu