docs: Add batch metadata retrieval example to README

Add concise documentation for get_filing_metadatas_batch() method:
- Shows example with multiple accession numbers
- Highlights optimization (2 API calls for 3 filings)
- Explains use case clearly without bloat

Updated both index.ipynb notebook and README.md

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: claude

README.md

@@ -178,6 +178,24 @@ Alternatively, you can also use any of these to get the same answer:
     metadatas = dl.get_filing_metadatas(RequestedFilings(ticker_or_cik="NFLX"))
     metadatas = dl.get_filing_metadatas(RequestedFilings(limit=1, ticker_or_cik="NFLX", form_type="10-Q"))
 
+Efficiently retrieve metadata for multiple specific accession numbers:
+
+Useful when you've downloaded filings in bulk and want metadata for
+specific accession numbers without making individual requests or
+fetching all filings for each company.
+
+``` python
+# Groups by CIK - makes only 2 API calls (one for AAPL, one for MSFT)
+metadatas = dl.get_filing_metadatas_batch([
+    "AAPL/0000320193-23-000077",
+    "AAPL/0000320193-24-000123",
+    "MSFT/0000950170-24-087843",
+])
+print(f"Retrieved {len(metadatas)} filings with {len(set(m.cik for m in metadatas))} API calls")
+```
+
+    Retrieved 3 filings with 2 API calls
+
 ## Download the HTML files
 
 After obtaining the Primary Document URL, for example from the metadata,

nbs/index.ipynb

@@ -315,6 +315,30 @@
     "```"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "Efficiently retrieve metadata for multiple specific accession numbers:\n",
+    "\n",
+    "Useful when you've downloaded filings in bulk and want metadata for specific accession numbers without making individual requests or fetching all filings for each company."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "# Groups by CIK - makes only 2 API calls (one for AAPL, one for MSFT)\n",
+    "metadatas = dl.get_filing_metadatas_batch([\n",
+    "    \"AAPL/0000320193-23-000077\",\n",
+    "    \"AAPL/0000320193-24-000123\",\n",
+    "    \"MSFT/0000950170-24-087843\",\n",
+    "])\n",
+    "print(f\"Retrieved {len(metadatas)} filings with {len(set(m.cik for m in metadatas))} API calls\")"
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {},
@@ -508,4 +532,4 @@
  },
  "nbformat": 4,
  "nbformat_minor": 4
-}
+}