Update more web/ui/README.md bits for new UI (#6197)

Signed-off-by: NJulius Volz <julius.volz@gmail.com>

web/ui/README.md

@@ -5,10 +5,8 @@ using the vfsgen library (c.f. Makefile).
 During development it is more convenient to always use the files on disk to
 directly see changes without recompiling.
 To make this work, remove the `builtinassets` build tag in the `flags` entry
-in `.promu.yml`, and then `make build`.
+in `.promu.yml`, and then `make build` (or build Prometheus using
+`go build ./cmd/prometheus`).
 
 This will serve all files from your local filesystem.
 This is for development purposes only.
-
-After making changes to any file, run `make assets` before committing to update
-the generated inline version of the file.