Add manual installation instructions to readme

[Can Olcer]

When running the installer with with rails stimulus:install on an existing project, it often fails to corretly update files to add the necessary lines of code, showing this error message: File unchanged! The supplied flag value not found!.

I think it would be helpful to add a section that explains the manual install to the readme, so when somebody installs it and sees that it failed to update a file correctly, they can just do it manually. In addition, we could add what it was supposed to do to the error message, something like:

File unchanged! The supplied flag value not found!  app/javascript/application.js

Wanted to add:
import "@hotwired/stimulus-loading"

to app/javascript/application.js, but couldn't parse the file correctly. Please add manually or retry.

Happy to look into this.

[David Heinemeier Hansson]

Please do investigate 👍

[Can Olcer]

Ok, I've looked into it and it looks like the weird error thrown by Thor (File unchanged! The supplied flag value not found!) is an issue with Thor that's already being worked on (see here).

Nevertheless, I created a PR that attempts to make the installation process more transparent for the user with this PR.