Fix installation instructions and dependencies

[BryanQuiceno]

This pull request adds CMake to the list of required dependencies and updates the installation instructions in the README.md file of the picom project. The absence of CMake was causing build errors related to libconfig version requirements.

Added cmake to the dependencies list under the Dependencies section.
Updated the list of required packages for Debian-based distributions to include cmake.

Explanation of the Issue

While attempting to compile picom, I encountered the following error:

objectivec

Dependency libconfig found: NO found 1.5 but need: '>=1.7'
Run-time dependency libconfig found: NO (tried pkgconfig and cmake)
src/meson.build:65: WARNING: Trying to clone and build libconfig as a subproject.

Executing subproject libconfig method cmake

libconfig| Did not find CMake 'cmake'
libconfig| Found CMake: NO

src/meson.build:75:1: ERROR: Unable to find CMake

The build process could not find libconfig version 1.7, and Meson attempted to clone and build libconfig as a subproject using CMake. However, since CMake was not installed, this process failed with the error ERROR: Unable to find CMake.

How the Issue Was Resolved

By installing CMake, Meson was able to successfully build libconfig as a subproject, resolving the version issue. This allowed the build process to proceed without needing to find libconfig version 1.7 in the system repositories.

Why These Changes Are Necessary

Including CMake as a required dependency ensures that other users will not encounter the same compilation error when building picom. This update improves the build instructions and helps prevent potential build failures related to libconfig and missing CMake.

[yshui]

cmake is not a hard dependency - it is only needed if you don't have libconfig 1.7. you should at least mention that in the changes.

[adamperkowski]

Duplicate of #1339