Added a lot of commentary into CMakeList.

This is just that I added a lot of comments to each new command in the CMakeList
file, because if I ever have to re-use this obscure and infrequently used skill,
I'll have a record written in my own words.

xcb_read/CMakeLists.txt

@@ -1,15 +1,43 @@
 project("XrandR")
 cmake_minimum_required(VERSION 3.18)
 
+# This is a small experiment in figuring out which compilers I want to use for
+# the complition. There's no reason to use Clang over GCC, but I was curious how
+# you specified that in the configuration.
+
+set(CMAKE_C_COMPILER "clang")
+set(CMAKE_CXX_COMPILER "clang++")
+
+# Packages are libraries that provide definitions for how to compile
+# well-known packages.  The XCB package is often installed someplace
+# peculiar, and the ECM (Extra CMake Modules) package knows how to
+# find it, but there are no modules to import from it, just the finder.
+
 find_package(ECM REQUIRED NO_MODULE)
 list(APPEND CMAKE_MODULE_PATH ${ECM_MODULE_PATH})
-
 find_package(XCB REQUIRED)
+
+# There are the definitions of the libraries needed to build the executable.
+
 link_libraries(${XCB_LIBRARIES})
 include_directories(${X11_INCLUDE_DIR})
 
+# The `list(APPEND ...` command does, well, it takes an existing string and
+# append new stuff to it.
+
 list(APPEND CMAKE_CXX_FLAGS "${CXXMAKE_C_FLAGS} -std=c++17 -I../src/include/ -g")
 
+# This is the final executable and the list of source files that are responsible
+# for building that executable. There can be multiple executables, and it can be
+# possible to describe which one you want from the command line.
+
 add_executable(xrandr src/xrandr.cpp)
 
+# Define the libraries that are necessary to link a binary. The link happens
+# only if it's a target mentioned in `add_executable()` (TK: Are there others
+# that provide executables?) Note that this has to come *after* the executable
+# target is provided, or CMake will complain that the target hasn't been found.
+
 target_link_libraries(xrandr ${X11_XCB_LIBRARIES})
+
+