docs(query): add readme
# Databend Query
Databend Query is a Distributed Query Engine at scale.
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+- `ast`, new version of sql parser based on `nom-rule`.
+- `catalog` contains structures and traits for catalogs management, `Catalog`, `Database`, `Table` and `TableContext`.
+- `codegen` is used to generate the arithmetic result type.
+- `config` provides config support for databend query.
+- `datablocks`, `Vec` collection, which encapsulates some common methods, and will be gradually migrated to `expressions`.
+- `datavalues`, the definition of each type of Column, which represents the layout of data in memory, will be gradually migrated to `expressions`.
+- `expression`, the new scalar expression framework with expression definition (AST), type checking, and evaluation runtime.
+- `formats`, the serialization and deserialization of data in various formats to the outside.
+- `functions`, scalar functions and aggregate functions, etc.
+- `functions-v2`, scalar functions and aggregate functions, etc., based on `expression`.
+- `legacy-parser`, the old parser, which will be replaced by ast, built with sqlparser-rs.
+- `menagement` for clusters, quotas, etc.
+- `pipeline` implements the scheduling framework for physical operators.
+- `planners` builds an execution plan from the user's SQL statement and represents the query with different types of relational operators.
+- `service` -> `databend-query`, the query service library of Databend.
+- `settings`, global and session level settings.
+- `storages` relates to table engines, including the commonly used fuse engine and indexes etc.
+- `streams` contains data sources and streams.
+- `users`, role-based access and control.
