📝: Update README.md [ci skip]

[jae-mon]

Summary

Added a note to point 2 of the header 'Get Started With Webpacker'.
For Rails 6, the line gem 'webpacker' is not needed in the gemfile, as
it is already added by default.

Other Information

Leaving in the line generates an error when bundle install is run:
"You cannot specify the same gem twice with different version requirements.
You specified: webpacker (>= 0) and webpacker (~> 4.0)"

[BookOfGreg]

Approved. I'd also welcome a PR that changes the default to assume webpack and have a note to add if less than Rails 6. I'd imagine there are more new Rails 6 projects today than 5 so.

[jae-mon]

Approved. I'd also welcome a PR that changes the default to assume webpack and have a note to add if less than Rails 6. I'd imagine there are more new Rails 6 projects today than 5 so.

Thank you for approving my PR. I will also work on the PR you suggested for changing the default to assume webpack.