Update readme

README.md

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-TODO:
+# Aggregate expression engines
 
-- Bool
-- Ints
-- Floats
+This repo contains Inngest's aggregate expression engine service, turning O(n^2) expression
+matching into O(n).
+
+It does this by:
+
+1. Parsing each expression whilst lifting literals out of expressions
+2. Breaking expressions down into subgroups (matching && comparators)
+3. Storing each group's comparator in a matching engine for fast lookups
+
+When an event is received, instead of matching the event against every expression, we instead
+inspect each matching engine to filter out invalid expressions.  This leaves us with a subset of
+expressions that are almost always matching for the event, simplifying the number of expressions
+to match against.
+
+Copyright Inngest 2024.