Dependencies missing from build instructions

The build instructions currently state that

You need Meson and Ninja (as well as Rust and Cargo) to build Fractal.

This completely ignores system dependencies. On a (nearly) fresh Fedora 27 box, jumping from build error to build error, I had to manually install the development files for

• GTK
• GDK
• GDK-PixBuf
• ATK
• Pango
• Cairo
• D-Bus
• OpenSSL

and possibly two or three others that I don't remember now.

These additional dependencies should be mentioned in the README, ideally with copy-and-pasteable dnf/apt commands for Fedora and Ubuntu that install all of them in a single step.