Add README for KQIR (apache#2304)

src/search/README.md

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+## KQIR: Kvrocks Query Intermediate Representation
+
+Here, *KQIR* refers to both
+- the multiple-level *query intermediate representation* for Apache Kvrocks, and
+- the *architecture and toolset* for the query optimization and execution.
+
+### Architecture
+
+![Architecture of KQIR](../../assets/KQIR.png)
+
+### Components
+
+- User Interface: both SQL and Redis Query syntax is supported to be the frontend language of KQIR
+  - SQL Parser: A parser that accepts an extended subset of MySQL syntax
+  - Redis Query Parser: A parser that accepts [Redis query syntax](https://redis.io/docs/latest/develop/interact/search-and-query/advanced-concepts/query_syntax/) (only DIALECT 2 or greater is planned to be supported)
+- KQIR: a multiple level query intermediate representation, currently including two levels (syntactical IR and planning IR)
+  - Syntactical IR: A high level IR that syntactically represents the query language
+  - Planning IR: A low level IR that represents plan operators for query execution
+- KQIR passes: analysis and transformation procedures on KQIR
+  - Semantic Checker: to check if there is any semantic errors in the IR
+  - Expression Passes: passes for query expressions, especially for logical expressions
+  - Numeric Passes: passes for numeric & arithmetic properties
+  - Plan Passes: passes on the plan operators
+  - Pass Manager: to manage the pass execution sequence and order
+  - Cost Model: to analyze the cost for the current plan, used by some plan passes
+- Plan Executor: a component for query execution via iterator model
+- Indexer: to perform the indexing for various types of fields during data changes