Use CMake option for asset dir instead of command-line flag

README

@@ -5,24 +5,25 @@ Dependencies:
 - libxml2 :: https://gitlab.gnome.org/GNOME/libxml2/
 - SDL2 :: https://www.libsdl.org/
 
-The build is handled with CMake (version 3.10 or later):
+The build is handled with CMake (version 3.10 or later).  You can
+specify a path to a directory containing the assets when configuring
+the build; alternatively it will default to the "assets" directory
+under the root of the project.
 
-	cmake -B build
+	cmake -Bbuild # -DASSETS_DIR="..."
 	cmake --build build
 
-The path to a directory containing the assets must be passed on the
-command line to run the game:
+You should then be able to run the game:
 
-   build/game/game ASSETS-DIR
+   build/game/game
 
 
 							Hot Reloading
 
 The engine supports hot reloading of the game code via the F5 key; set
-the HOTRELOAD option on in CMake to enable it. You also have to run an
-executable built from the engine rather than the game if using this
-feature.
+the HOTRELOAD option on in CMake to enable it.  You also have to run
+the engine executable rather than the game one if using this feature.
 
-	cmake -B build -D HOTRELOAD=ON
+	cmake -Bbuild -DHOTRELOAD=ON
 	cmake --build build
-	build/engine/engine assets/
+	build/engine/engine