Add replay specification

REPLAY.md

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+# Specification of bgammon.org replay file
+
+Replays are stored as .match files.
+
+## Match format (.match)
+
+A .match file contains one or more games concatenated together.
+
+The games are prefixed with a table listing the index of each game.
+
+### Index table
+
+The index table consists of one or more lines in the following format:
+
+`bgammon-replay <index>`
+
+Games are in chronological order.
+The index specifies the index of the first byte of the first line of a game.
+The index is always eight digits with leading zeroes.
+
+### Game
+
+#### Metadata
+
+The first line of the file specifies the metadata.
+
+`i <player1> <player2> <total> <score1> <score2> <winner> <points> <acey>`
+
+#### Index table
+
+The index table consists of one or more lines in the following format:
+
+`g <player> <index>`
+
+The index specifies the index of the first byte of each line corresponding to each turn in the game.
+The index is always eight digits with leading zeroes.
+
+#### Events
+
+##### Double
+
+Accepted:
+
+`d 2 1`
+
+Declined:
+
+`d 2 0`
+
+##### Roll and move
+
+Moves are always specified from player 1's perspective.
+
+`r 5-3 13/8 24/21`