Add the "assets" make target and change directory note in readme (#7908)

* add the "assets" make target and change directory note in readme

Signed-off-by: Luke Chen <showuon@gmail.com>

* address reviewer's comment

Signed-off-by: Luke Chen <showuon@gmail.com>

README.md

@@ -93,6 +93,7 @@ The Makefile provides several targets:
   * *format*: format the source code
   * *vet*: check the source code for common errors
   * *docker*: build a docker container for the current `HEAD`
+  * *assets*: build the new experimental React UI
 
 ## React UI Development
 

web/ui/react-app/README.md

@@ -26,6 +26,8 @@ The React UI depends on a large number of [npm](https://www.npmjs.com/) packages
 
 Yarn consults the `package.json` and `yarn.lock` files for dependencies to install. It creates a `node_modules` directory with all installed dependencies.
 
+**NOTE**: Remember to change directory to `web/ui/react-app` before running this command and the following commands.
+
 ## Running a local development server
 
 You can start a development server for the React UI outside of a running Prometheus server by running: