Update manifest-tasks.mdx

Add cpp.

Author: crandmck

docs/manifest/manifest-tasks.mdx

@@ -106,7 +106,23 @@ await read('my-c2pa-file.jpg', 'image/jpeg');
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   <TabItem value="cpp" label="C++">
-    This is how to read a manifest using C++. 
+Use the `read_file` function to read C2PA data from the specified file. This function examines the specified asset file for C2PA data and its return value is a JSON report if it finds C2PA data. If there are validation errors, the report includes a `validation_status` field. Exceptions are thrown on errors.
+
+```cpp
+auto json_store = C2pa::read_file("<ASSET_FILE>", "<DATA_DIR>")
+```
+
+Where:
+
+- `<ASSET_FILE>`- The asset file to read; The file must be one of the [supported file formats](#supported-file-formats).
+- `<DATA_DIR>` - Optional path to data output directory; If provided, the function extracts any binary resources, such as thumbnails, icons, and C2PA data into that directory. These files are referenced by the identifier fields in the manifest store report.
+
+For example:
+
+```cpp
+auto json_store = C2pa::read_file("work/media_file.jpg", "output/data_dir")
+```
+
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