Not an umlaut, a diaeresis - it shows that the letter has its own sound (unlike the word "coop", for example).
You'd generally use it for proper nouns (like Brontë or Boötes) and rare loanwords - it doesn't seem very useful for common dictionary words, since anyone who can understand the word itself probably knows how to pronounce it.
You'd generally use it for proper nouns (like Brontë or Boötes) and rare loanwords - it doesn't seem very useful for common dictionary words, since anyone who can understand the word itself probably knows how to pronounce it.