Missing libssl-dev dependency

[GustavoLR548]

Hi, just letting you know that I have been trying to install this signalling server recently, and I faced the following issue when running ```cargo build --release´``

error: failed to run custom build command for `openssl-sys v0.9.103`

I later found out that's because I needed to download the libssl-dev package to my ubuntu. I don't know if this is the standard for other distributions, but I thought it would be interesting to mention in the readme since it is required to actually build the server.

[drwhut]

Yeah, this is normal for using the OpenSSL library in Rust. From what I recall, the error message actually says to install your distribution's ssl-dev library, but I will consider adding a note in the README about this as well.