[SUP-1802] Update React example app readme

[luke-belton]

SUP-1802

Goal

Update readme to include instructions on running a development server.

Design

Make it clear to users of the example app that they can get live-reloading of the code

Changeset

README.md

Testing

n/a

[github-actions]

@bugsnag/browser bundle size diff

	Minified	Minfied + Gzipped
Before	42.67 kB	13.08 kB
After	42.67 kB	13.08 kB
±	No change	No change

code coverage diff

Coverage values did not change👌.

Total:

Lines	Branches	Functions	Statements
0%(+0%)	0%(+0%)	0%(+0%)	0%(+0%)

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