Update Readme for Instructions for running lighthouse cli for site with authentication.

[mikerhyssmith]

The current instructions in the documentation for running lighthouse against a site where you need to login first are:

• Run chrome-debug
• navigate to and log in to your site
• in a separate terminal tab lighthouse http://mysite.com

However, from what I can see for this to work when you run the lighthouse command you would need to add --port and the port number the chrome-debug instance is running on otherwise it opens a new instance of Chrome.

Happy to open a PR if this is the best way but it might be there's a better way of doing this !

[ebidel]

Correct. The chrome launcher was changed to select a random port rather than defaulting to 9222.

PR would be great which adds the --port!

cc @samccone @paulirish another case where a rando port makes things harder.

[brendankenny]

yeah, #2259 would fix a lot of those cases but not this one