Updated Structure of the code (markdown)

Structure-of-the-code.md

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+osxphotos is built around a set of classes that each handle a particular aspect of the Photos library:
+
+* PhotosDB: reads the Photos library database and extract all relevant metadata about each photo.  This is starting point for any use of osxphotos.
+* PhotoInfo: represents an individual photo in the library and all associated metadata.
+* AlbumInfo: represents an album in the library.
+* FolderInfo: represents a folder in the library.
+* PlaceInfo: represents the reverse geolocation info (e.g. the address or place name) associated with a photo.
+* ExifTool: provides an interface to the [exiftool](https://exiftool.org/) tool for manipulating EXIF information associated with a photo.
+
+The package code is organized in osxphotos thusly:
+
+```
+osxphotos
+├── __init__.py              # this is what gets read by "import osxphotos"
+├── __main__.py              # the command line interface, what gets executed with python3 -m osxphotos
+├── _applescript             # contains a copy of pyapplescript from https://github.com/rdhyee/py-applescript
+├── _constants.py            # constants used by the other modules
+├── _version.py              # version string
+├── albuminfo.py             # AlbumInfo and FolderInfo classes
+├── datetime_formatter.py    # utility methods for formatting datetime objects
+├── exiftool.py              # ExifTool class
+├── photoinfo.py             # PhotoInfo class
+├── photosdb.py              # PhotosDB class 
+├── placeinfo.py             # PlaceInfo class
+├── template.py              # constants used by the PhotoInfo.render_template() method
+├── templates                # contains mako templates, see https://www.makotemplates.org/
+│   └── xmp_sidecar.mako
+└── utils.py
+```
+