inform user about the location of `myst.yml` when not found in `.`

[parmentelat]

trafficstars

I mistakenly ran

myst start

in the wrong folder (in the parent of where I should have)

what happened then is that myst has been searching a myst.md up in my filesystem, and unluckily found one .. in my homedir - a leftover of a mistaken myst init probably

in any case what happened next was that myst has started searching my whole homedir, it did find quiet a few _toc.yml and issued a zillion of error messages

in order to shorten the troubleshooting in this case, it would have helped me greatly if I had seen first off a message saying

using /myhome/myst.yml probably for keeping the noise low, this is helpful only when said config is not in .