Create usage_analytics.md

Author: williebsweet

docs/usage_analytics.md

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+# Usage Analytics & Data Privacy
+
+data-diff collects anonymous usage data to help our team improve the tool and to apply development efforts to where our users need them most.
+
+We capture two events: one when the data-diff run starts, and one when it is finished. No user data or potentially sensitive information is or ever will be collected. The captured data is limited to:
+
+- Operating System and Python version
+- Types of databases used (postgresql, mysql, etc.)
+- Sizes of tables diffed, run time, and diff row count (numbers only)
+- Error message, if any, truncated to the first 20 characters.
+- A persistent UUID to indentify the session, stored in `~/.datadiff.toml`
+
+To disable, use one of the following methods:
+
+* **CLI**: use the `--no-tracking` flag.
+* **Config file**: set `no_tracking = true` (for example, under `[run.default]`)
+* **Python API**:
+    ```python
+    import data_diff
+    # Invoke the following before making any API calls
+    data_diff.disable_tracking()
+    ```