Document prerequisites for download-experimental-build

[eps1lon]

Summary

Following

react/packages/react-devtools-extensions/README.md

Lines 23 to 26 in 461cd84

	To use the latest build from CI, run the following command from the root of the repository:
	```sh
	./scripts/release/download-experimental-build.js
	```

./scripts/release/download-experimental-build.js is not runnable since it's not mentioned that one has the install the nested workroot first.

Test Plan

• Steps work in Ubuntu 20.04
• CI green

[sizebot]

Comparing: f227e7f...d2bbb86

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-stable/react-dom/cjs/react-dom.production.min.js	=	122.83 kB	122.83 kB	=	39.52 kB	39.52 kB
oss-experimental/react-dom/cjs/react-dom.production.min.js	=	129.33 kB	129.33 kB	=	41.59 kB	41.58 kB
facebook-www/ReactDOM-prod.classic.js	=	408.70 kB	408.70 kB	=	75.80 kB	75.80 kB
facebook-www/ReactDOM-prod.modern.js	=	396.96 kB	396.96 kB	=	73.86 kB	73.86 kB
facebook-www/ReactDOMForked-prod.classic.js	=	408.71 kB	408.71 kB	=	75.80 kB	75.80 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

Generated by 🚫 dangerJS against d2bbb86