docs(nodejs): explain opening existing LangChain tables

nodejs/README.md

@@ -30,6 +30,27 @@ const results = await table.vectorSearch([0.1, 0.3]).limit(20).toArray();
 console.log(results);
 ```
 
+### Use an Existing Table with LangChain
+
+When wrapping an existing table with `@langchain/community`, open the table
+with LanceDB and pass the resulting table handle to LangChain. The LangChain
+`uri` and `tableName` options are used when creating a table; they do not open
+an existing table for search.
+
+```javascript
+import { LanceDB as LangChainLanceDB } from "@langchain/community/vectorstores/lancedb";
+import * as lancedb from "@lancedb/lancedb";
+
+const db = await lancedb.connect("data/sample-lancedb");
+const table = await db.openTable("my_table");
+const vectorStore = new LangChainLanceDB(embeddings, { table });
+
+const results = await vectorStore.similaritySearchVectorWithScore(
+  queryVector,
+  5,
+);
+```
+
 The [quickstart](https://docs.lancedb.com/quickstart/) contains more complete examples.
 
 ## Development