Write README with usage example (#26)

* Add README with usage example

Document project purpose, installation, and provide a working code example
that demonstrates parsing SQL and generating EXPLAIN output.

* Fix import path to sqlc-dev/doubleclick

* Add JSON output example and remove License section

* Reorder output: JSON first, then EXPLAIN

---------

Co-authored-by: Claude <[email protected]>

Author: kyleconroy

README.md

@@ -0,0 +1,104 @@
+# doubleclick
+
+A ClickHouse SQL parser written in Go. Parses ClickHouse SQL syntax into an Abstract Syntax Tree (AST) and generates EXPLAIN output matching ClickHouse's format.
+
+## Installation
+
+```bash
+go get github.com/sqlc-dev/doubleclick
+```
+
+## Usage
+
+```go
+package main
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"strings"
+
+	"github.com/sqlc-dev/doubleclick/parser"
+)
+
+func main() {
+	sql := `SELECT id, name FROM users WHERE active = 1 ORDER BY created_at DESC LIMIT 10`
+
+	stmts, err := parser.Parse(context.Background(), strings.NewReader(sql))
+	if err != nil {
+		panic(err)
+	}
+
+	// Serialize to JSON
+	jsonBytes, _ := json.MarshalIndent(stmts[0], "", "  ")
+	fmt.Println(string(jsonBytes))
+
+	// Or print EXPLAIN AST output (matches ClickHouse format)
+	fmt.Println(parser.Explain(stmts[0]))
+}
+```
+
+JSON output:
+
+```json
+{
+  "selects": [
+    {
+      "columns": [
+        { "parts": ["id"] },
+        { "parts": ["name"] }
+      ],
+      "from": {
+        "tables": [
+          { "table": { "table": { "table": "users" } } }
+        ]
+      },
+      "where": {
+        "left": { "parts": ["active"] },
+        "op": "=",
+        "right": { "type": "Integer", "value": 1 }
+      },
+      "order_by": [
+        {
+          "expression": { "parts": ["created_at"] },
+          "descending": true
+        }
+      ],
+      "limit": { "type": "Integer", "value": 10 }
+    }
+  ]
+}
+```
+
+EXPLAIN output:
+
+```
+SelectWithUnionQuery (children 1)
+ ExpressionList (children 1)
+  SelectQuery (children 4)
+   ExpressionList (children 2)
+    Identifier id
+    Identifier name
+   TablesInSelectQuery (children 1)
+    TablesInSelectQueryElement (children 1)
+     TableExpression (children 1)
+      TableIdentifier (children 1)
+       Identifier users
+   Function equals (children 1)
+    ExpressionList (children 2)
+     Identifier active
+     Literal UInt64_1
+   ExpressionList (children 1)
+    OrderByElement (children 1)
+     Identifier created_at
+   Literal UInt64_10
+```
+
+## Features
+
+- Parses SELECT, INSERT, CREATE, DROP, ALTER, and other ClickHouse statements
+- Handles ClickHouse-specific syntax (Array types, PREWHERE, SAMPLE, etc.)
+- Supports JOINs, subqueries, CTEs, window functions, and complex expressions
+- Generates JSON-serializable AST nodes
+- Produces EXPLAIN AST output matching ClickHouse's format