Add note about heap allocator (#4)

Author: finger563

render_markdown.py

@@ -182,6 +182,7 @@ def fmt_row(label: str, key: str, show_pct: bool) -> str:
         *rows,
         "",
         "<sub>FLASH uses app .bin size or json2 flash sum. RAM sums DRAM+IRAM via idf_size. Percentages shown when totals are available.</sub>",
+        "<sub>DRAM/IRAM usage does not include memory used by the heap allocator at runtime.</sub>",
         "<sub>This report was generated by [esp-idf-size-delta](https://github.com/esp-cpp/esp-idf-size-delta).</sub>",
         "",
         f"<!-- esp-idf-size-delta:{args.app_name} -->",