Mention a fallback for chruby users if project doesn't have .ruby-version

[st0012]

Motivation

For chruby users, working on projects without .ruby-version (e.g. /ruby/irb) would cause Ruby LSP fail to start. A workaround for that is to have .ruby-version at a level above (e.g. /ruby/.ruby-version). But this information is not widely known.

Implementation

Add note to the version manager section to explain this.

[vinistock]

Do you think it makes sense to put this in a new markdown file for version managers?

Then we can aggregate all information there for different options.

[st0012]

Do we have information for other managers?
I personally want to keep these kind of info at README until we're out of space, because it's very critical for users' experience with the tool and should be the easiest to find.

[vinistock]

There's interest from people using rtx #668. How about we add a link from the README?

Something like

For version manager specific instructions and tips, see [blah]()

[vinistock]

Can we move this into a chruby section in the VERSION_MANAGERS.md file?

[st0012]

Do you agree with moving the entire custom activation section there too?
Currently VERSION_MANAGERS is an extension of it, so I feel a bit weird putting non-custom activation content here.

It'll be like

...

To make sure that the Ruby LSP can find the version manager scripts, make sure that they are loaded in the shell's
configuration script (e.g.: ~/.bashrc, ~/.zshrc) and that the SHELL environment variable is set and pointing to the
default shell.

For manager-specific notes and how to set up custom activation for managers that are not listed above, please see [version managers](VERSION_MANAGERS.md)

[vinistock]

Yes, but let's at least mention custom activation in the README and link to the file, so that it's discoverable.