readme usage: added python example

README.md

@@ -1,3 +1,10 @@
+## Install
+
+```sh
+pip3 install --user freesurfer-volume-reader
+freesurfer-volume-reader --help
+```
+
 ## Usage
 
 ```sh
@@ -11,6 +18,16 @@ or
 freesurfer-volume-reader /my/freesurfer/subjects
 ```
 
+or
+
+```python
+from freesurfer_volume_reader import find_hippocampal_volume_files, \
+                                     read_hippocampal_volume_file_dataframe
+
+for volume_file_path in find_hippocampal_volume_files('/my/freesurfer/subjects'):
+    print(read_hippocampal_volume_file_dataframe(volume_file_path))
+```
+
 ## Tests
 
 ```sh