Add extraction example to CLI documentation

README.md

@@ -169,6 +169,13 @@ Here are some Bash examples which illustrate getting and setting tag information
   tageditor get --files /some/dir/*.mkv
   ```
 
+* Extracts the cover of the specified (Opus) file:  
+  ```
+  tageditor extract cover --output-file the-cover.jpg --file some-file.opus
+  ```
+
+    - No conversion is done by the tag editor. This command assumes that the cover is a JPEG image.
+    - The extraction works for other fields as well. For Matroska attachments one needs to use `--attachment`.
 
 * Displays technical information about all \*.m4a files in the specified directory:  
   ```

application/main.cpp

@@ -185,6 +185,7 @@ int main(int argc, char *argv[])
         "saves the value of the specified field (eg. cover or other binary field) or attachment to the specified file or writes it to stdout if no "
         "output file has been specified");
     extractFieldArg.setSubArguments({ &fieldArg, &attachmentArg, &fileArg, &outputFileArg, &verboseArg });
+    extractFieldArg.setExample(PROJECT_NAME " extract cover --output-file the-cover.jpg --file some-file.opus");
     extractFieldArg.setCallback(std::bind(
         Cli::extractField, std::cref(fieldArg), std::cref(attachmentArg), std::cref(fileArg), std::cref(outputFileArg), std::cref(verboseArg)));
     // export to JSON