Add basic api documentation

README.md

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 A memory-based caching layer for beatmap-rank lookups which cannot be easily optimised as a database/SQL level.
+
+Intended to handle queries which iterate over large sections of (already indexed) scores, where the overhead of counting rows becomes an issue.
+
+# Query API
+
+`/ranklookup?beatmapId={$beatmapId}&score={$score}&rulesetId={$mode}`
+
+`$beatmapId` - The beatmap ID to lookup
+`$score` - The achieved total score value
+`$mode` - The ruleset ID (0..3)
+
+# Response
+
+An zero-index integer denoting the rank in the leaderboard for the provided score.
+
+# TODO
+
+- Currently this is only useful for global leaderboards. Mod filters cannot be applied, for instance.
+- After an initial query, the queried beatmap will be tracked permanently. There is no cache expiry, so the potential of saturating available memory is real. LRU expiry should be implemented.
+- More correctly handling top 50 lookups where score collisions are feasible (need to fallback to ID).
+