README.md updated with prerequisites for building from source code

[idelac3]

Please include the following oneliner

apt install autoconf autoconf-archive pkg-config libimlib2 libimlib2-dev libxcomposite1 libxcomposite-dev libxext6 libxext-dev libxfixes3 libxfixes-dev libxinerama1 libxinerama-dev

under dependencies section, in the README.md file.

[N-R-K]

If you're an end user you should just be able to do apt install scrot (i think). For developers, we have some instruction in CONTRIBUTING.md already:

scrot/CONTRIBUTING.md

Lines 10 to 13 in 5ed4a99

	You will want to install the dependencies listed in the `Building` section of
	[README.md](README.md). Look for the text `install_script` in the
	[.cirrus.yml](.cirrus.yml) CI definitions for example package manager
	invocations on many operating systems.

And the mentioned .cirrus.yml file contains dependency list for more than just debian.

Personally, I don't see any reason to special case debian here. Especially since different distros have different naming/packing conventions and so it's not necessarily transferable across distros.