Rename the Makefile, add usage note at the top

I had named it makefile_web" in an effort to communicate to the user
that the Make-based build path is only for web builds. It's annoying to
type all that out, though, and it doesn't seem like other projects
follow this convention. I'll just put a usage note at the top. It's not
like the Makefile can be mis-used to make a non-web version. There is no
footgun here.

Makefile

@@ -1,3 +1,6 @@
+# This script produces a web build. If you aren't trying to do that, it is
+# entirely useless to you.
+
 SRC_DIR = ./src
 SRCS := $(wildcard $(SRC_DIR)/**)