Add a usage disclaimer in README.org

[daviwil]

This project is receiving a lot of excellent PRs! Since we're still in the early phases of figuring out the right way to structure everything, it'd probably be worthwhile to add a disclaimer to let potential users know that this config could break for them on their next pull due to things changing quickly.

We shouldn't discourage people from trying the configuration, though!

[jeastman]

I figure it is easier to sort out wording here than in a series of pull requests, so... how is this?

Please note: rational-emacs is under active development. While the intent
of this project is to provide a stable Emacs configuration for use by
anyone, it is not yet stable enough for everyday or production use.  The
rapid pace of changes could cause your configuration to break on your next
pull of the repository. If you are not expecting or prepared to encounter
such issues, we would recommend you wait for things to stabilize a bit
before using.

[ajxn]

Maybe a suggestion or five how one could recover from problems like that?
Like

• Which switch to use to get Emacs to debug init.el file and comment out the problem? (emacs --debug-init)
• Which switch to start without reading init.el configuration? (emacs --no-init-file)
• That Cemacs2 could be a solution for that? (???)
• How to back to previous version with git (???)
• How to generate HTML, PDF or ODT documents from the README.org file for reference? (C-c C-e h h and C-c C-e l p or C-c C-e o o in the README.org file).

And how one could file a bug report?

[daviwil]

@jeastman Sounds good to me!

@ajxn Great suggestions! Since this config might be tried by beginners, it'd be nice to have a longer "Troubleshooting" section or document that could help them diagnose issues they run into.

[daviwil]

Fixed by #55.