Adjust readme to refer to new dev server

It does not serve plugins so added a note that you should serve plugin
files directly

Change-Id: I4a91cbe2b4e7004ac699e013087aa792aebf1c3b

README.md

diff --git a/README.md b/README.md
index b75a1bf..76df640 100644
--- a/README.md
+++ b/README.md
@@ -28,10 +28,15 @@
 
 ### How to use
 
-1. Start the go server to host all assets locally
+1. For Gerrit core development, start the local gerrit dev server to host app code locally
 ```sh
-./polygerrit-ui/run-server.sh
+yarn start
 ```
+Or if you are developing a plugin, serve your plugin locally via any means. Ex:
+```sh
+npx http-server -c-1 --cors
+```
+
 2. Go to any gerrit sites, enable the extension by click the icon
 3. You should see this red notice show up in the bottom right of the page (`Gerrit dev helper is enabled`),
 and now your gerrit assets should be loaded from local server