Add exampele3 with pynautobot

Author: dgarros

examples/example3/README.md

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+
+# Example 3
+
+This is a simple example to show how DiffSync can be used to compare and synchronize data with a remote system like via a REST API like Nautobot.
+
+For this example, we have a shared model for Region and Country defined in `models.py`.
+A Country must be associated with a Region and can be part of a Subregion too.
+
+The comparison and synchronization of dataset is done between a local JSON file and the [public instance of Nautobot](https://demo.nautobot.com).
+
+
+## Install the requirements
+
+to use this example you must have some dependencies installed, please make sure to run 
+```
+pip install -r requirements.txt
+```
+
+## Try the example
+
+The first time a lot of changes should be reported between Nautobot and the local data because by default the demo instance doesn't have the subregion define.
+After the first sync, the diff should show no difference. 
+At this point, Diffsync will be able to identify and fix all changes in Nautobot. You can try to add/update or delete any country in Nautobot and DiffSync will automatically catch it and it will fix it with running in sync mode.
+
+```
+### DIFF Compare the data between Nautobot and the local JSON file.
+main.py --diff
+
+### SYNC Update the list of country in Nautobot.
+main.py --sync
+```
+