Add apological comments to Makefile.

Makefile

@@ -1,7 +1,15 @@
+# Half-baked Makefile; mostly for reference of commands.
+# Requires two tools, web-ext and crx.
+# npm install -g web-ext crx
+
+.PHONY: build
 build: fx-build cr-build
 
+.PHONY: fx-build
 fx-build:
 	web-ext -s ./extension -a ./dist build
 
+# Note: crx will overwrite a pre-existing output file.
+.PHONY: cr-build
 cr-build:
 	crx pack ./extension -o ./dist/youtube-cinema.crx -p .chrome-extension-key.pem