Add documentation to README to know when codespace is built?

[xronosuf]

In the README there is the following description about loading the initial codespace for new users:

It would be helpful if there was a comment somewhere in here that told users when to know that the codespace has been fully loaded. I typically tell people something like "I wait until the buttons in the bottom right appear, like 'serve' and 'extras', then wait another 10 or so sections to be safe." If I were new to the project and didn't know what Ximera was, I'd be awful paranoid about if I waited long enough - especially if something didn't work how I expected (which would happen a lot to a new user messing around) at some point.

This was found/reported as part of the Ximera Project Audit