Add JavaScript config example to README

[aarongoldenthal]

Update README to include additional details and example with JavaScript config file, including how it can be used to dynamically update config. Based on question from #193.

[aarongoldenthal]

The GitHub actions failures seem to be caused by ubuntu-latest moving to v22.04, per pa11y/pa11y#651 and pa11y/pa11y#662 (they tests all pass if pinning to ubuntu-20.04, although that's obviously not a long-term fix).

[danyalaytekin]

@aarongoldenthal Hello again! Thanks for this contribution too. As I mentioned I'd like to merge it into main before we publish v3.1.0. I've made two commits with tweaks which are described by their commit names above - please let me know whether you think they're ok or would like me to revert them, or for us to make further changes.

P.S. About your pinning comment - as you've probably noticed, I have settled for the pinning for now, for these minor-level updates, although I'm working to describe the chromeLaunchConfig workaround consistently across repos. Normal *-latest service will return with the next major version of each project.

[aarongoldenthal]

@aarongoldenthal Hello again! Thanks for this contribution too. As I mentioned I'd like to merge it into main before we publish v3.1.0. I've made two commits with tweaks which are described by their commit names above - please let me know whether you think they're ok or would like me to revert them, or for us to make further changes.

@danyalaytekin LGTM