document the make-sfx.sh fast option

docs/devnotes.md

@@ -258,6 +258,7 @@ then build the sfx using any of the following examples:
 
 ```sh
 ./scripts/make-sfx.sh           # regular edition
+./scripts/make-sfx.sh fast      # build faster (worse js/css compression)
 ./scripts/make-sfx.sh gz no-cm  # gzip-compressed + no fancy markdown editor
 ```
 

scripts/make-sfx.sh

@@ -6,6 +6,8 @@ help() { exec cat <<'EOF'
 
 # optional args:
 #
+# `fast` builds faster, with cheaper js/css compression
+#
 # `clean` uses files from git (everything except web/deps),
 #   so local changes won't affect the produced sfx
 #