Update build instructions

[ZackeryRSmith]

Windows, Mac, and Linux building has now been significantly changed, for better (or for worse). Developers compiling bongocat-plus no longer need to fuss about installing SFML, as it is installed and compiled for you on the first build of bongocat-plus.

This may seem redundant to some, and I would probably think that to, but after trying to compile bongocat-plus on windows I approve of this. I spent nearly 3 hours trying to compile bongocat-plus on windows with little luck. The issue was with SFML, and linking.

I do plan on making this optional for Mac, and Linux users if SFML is already installed on the system. Regardless, the build instructions on the readme are outdated.

[ZackeryRSmith]

I do plan on making this optional for Mac, and Linux users if SFML is already installed on the system. Regardless, the build instructions on the readme are outdated.

I've pinned this issue because to me it's now a seriously annoying problem. Unlike SFML on windows, the SFML dependencies to compile must be installed manually. If on MacOSX or Linux, building SFML from ground up automatically won't be supported.

I would instead please ask to install SFML through the package manager of your choice.