Add list of dependencies required to build awesome in README

[Blu3cr0ss]

The problem is that when I want to build awesome I have to spend time finding and installing dependencies. I made a list of required and optional dependencies:

# required
gcc
lua
git
cmake
imagemagick
pkgconf
# optional
asciidoctor
ldoc

I think it would be a good idea to add a command to install these dependencies in the README in the building and installation section.
For example:

Arch Linux

pacman -S gcc lua git cmake imagemagick pkgconf

etc...

[actionless]

it's there: https://awesomewm.org/apidoc/documentation/10-building-and-testing.md.html