What information, neccessary to the quest, is revealed in Rivendell in the chapter "A Short Rest"?

The rune-stones are discovered and supply (through a riddle) the instructions for finding the key hole in the door.

What are moon-letters?" asked the hobbit full of excitement. He loved maps, as I have told you before; and he also liked runes and letters and cunning handwriting, though when he wrote himself it was a bit thin and spidery.

"Moon-letters are rune-letters, but you cannot see them," said Elrond, "not when you look straight at them. They can only be seen when the moon shines behind them, and what is more, with the more cunning sort it must be a moon of the same shape and season as the day when they were written. The dwarves invented them and wrote them with silver pens, as your friends could tell you. These must have been written on a midsummer's eve in a crescent moon, a long while ago."

"What do they say?" asked Gandalf and Thorin together, a bit vexed perhaps that even Elrond should have found this out first, though really there had not been a chance before, and there would not have been another until goodness knows when.

"Stand by the grey stone when the thrush knocks," read Elrond, "and the setting sun with the last light of Durin's Day will shine upon the key-hole."