Improve upgrade instructions for Bootsnap
Improve upgrade instructions for Bootsnap

# Summary
Provide more clear instructions to add Bootsnap to the Gemfile, when upgrading for 5.1 to 5.2.
If the user decides to use it, but forget to add it to the gem file, the error below may pop-up.
```ruby
Traceback (most recent call last):
	3: from bin/rails:3:in `<main>'
	2: from bin/rails:3:in `require_relative'
	1: from <APP_BASE_FOLDER>/config/boot.rb:4:in `<top (required)>'
<APP_BASE_FOLDER>/config/boot.rb:4:in `require': cannot load such file -- bootsnap/setup (LoadError)
```
rails/rails,georgeclaghorn/rails,georgeclaghorn/rails,kddeisz/rails,Edouard-chin/rails,yawboakye/rails,Edouard-chin/rails,Erol/rails,esparta/rails,kddeisz/rails,kmcphillips/rails,BlakeWilliams/rails,rails/rails,yahonda/rails,georgeclaghorn/rails,MSP-Greg/rails,rails/rails,kmcphillips/rails,yawboakye/rails,yawboakye/rails,Erol/rails,kmcphillips/rails,yawboakye/rails,yahonda/rails,georgeclaghorn/rails,esparta/rails,Erol/rails,BlakeWilliams/rails,rails/rails,BlakeWilliams/rails,MSP-Greg/rails,MSP-Greg/rails,yahonda/rails,esparta/rails,yahonda/rails,kmcphillips/rails,Erol/rails,kddeisz/rails,MSP-Greg/rails,esparta/rails,Edouard-chin/rails,BlakeWilliams/rails,Edouard-chin/rails,kddeisz/rails