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

@@ -0,0 +1,44 @@
+# 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)/**)
+
+# Patch these to select a different build profile or target
+# The target shouldn't change any time soon. WASM64, I guess. Other targets
+# aren't aimed at the web, so you shouldn't be using this makefile.
+CARGO_TARGET := wasm32-unknown-unknown
+CARGO_PROFILE := wasm-release
+
+.PHONY: clean full-clean web tarball
+
+web: out/boids.js out/boids_bg.wasm out/index.html
+
+tarball: boids_web_root.tar
+
+boids_web_root.tar: out/boids.js out/boids_bg.wasm out/index.html
+	tar -caf $@ $^
+
+target/$(CARGO_TARGET)/$(CARGO_PROFILE)/another-boids-in-rust.wasm: $(SRCS) Cargo.lock Cargo.toml
+	cargo build --profile $(CARGO_PROFILE) --target $(CARGO_TARGET)
+
+out:
+	mkdir $@
+
+# Both the JS and WASM files are generated by the wasm-bindgen call, so both
+# get to be on the target half of this recipe.
+out/boids.js out/boids_bg.wasm &: target/$(CARGO_TARGET)/$(CARGO_PROFILE)/another-boids-in-rust.wasm | out
+	wasm-bindgen --no-typescript --target web --out-dir ./out/ --out-name boids target/$(CARGO_TARGET)/$(CARGO_PROFILE)/another-boids-in-rust.wasm
+
+out/index.html: www/index.html
+	cp -a $< $@
+
+# Clean the web build, but not the Cargo cache. Cargo handles it's own caching
+# and I don't want to obliterate it all the time.
+clean:
+	rm -rf out/ boids_web_root.tar
+
+# Delete everything, including the Cargo build cache. In case someone needs
+# this, I guess.
+full-clean: clean
+	cargo clean