Update README.md

I had to run `npm run prebuild` to setup `build.ninja` in the out dir. Without it I wasn't able to run any of the other build options.

Author: sbuggay

README.md

@@ -19,6 +19,11 @@ This repository is a fork of [ChromeDevTools/devtools-frontend](https://github.c
 
 ### Build-and-run options
 
+Run prebuild to set up the out dir:
+```
+npm run prebuild
+```
+
 1. Build continuously with a file watcher:
 
     ```sh