Upgrade the upgrade (instructions)

[jaredbeck]

After a major-version upgrade (eg. webpacker 3 to 4), I found it necessary to run the webpacker:install task, to update (replace) files such as babel.config.js

I barely know what I'm doing, so if there's a better way, please correct me.

[danielpowell4]

@jaredbeck here is a bit more thorough guide:

https://github.com/rails/webpacker/blob/master/docs/v4-upgrade.md

My guess is that when stable 4 comes out there will be a decent how-to release post of some sort. With Rails 6 just around the corner, I assume it's pretty close!

More changes proposed here: #1929

[jaredbeck]

@jaredbeck here is a bit more thorough guide:

https://github.com/rails/webpacker/blob/master/docs/v4-upgrade.md

Cool. I didn't realize that existed, thanks. Should we link to it from the readme?

[gauravtiwari]

Thanks, @jaredbeck will merge #1929 which includes this change.