Add note about gtk2hs-buildtools

[noughtmare]

I believe stack install gtk2hs-buildtools is needed before installing Yi so it would be very useful to have that in the installation instructions.

[ghost]

It was skipped earlier because:

1. Users can choose to use vty (and not pango) in which case they don't need to install the package
2. The actual package can be distro specific (instead of just using the stack version)

[noughtmare]

I think point 1 makes it much harder for beginners, they don't always now how to disable pango. Also we could just write something like "if you're compiling the pango frontent (which is the default) ..."
I don't really understand point 2, isn't it always possible to install gtk2hs-buildtools with stack? If not, we should just list the packages for common distros (like ubuntu, debian and fedora) and mac and windows.

[ghost]

It used to be cabal install -fvty yi. And vty was the default frontend at the time, so that made it easy for beginners. Things have changed a lot since then. The whole documentation website needs an overhaul. Probably some animated snapshots, and more importantly documentation of the high level buffer functions. Will update it soon™ (probably once we finalize the space leak problems).