[README.md] center SS

Author: chrissimpkins

README.md

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 `fdiff` is a Python command line comparison tool for differences in the OpenType table data between font files.  The tool provides cross-platform support on macOS, Windows, and Linux systems with a Python v3.6+ interpreter.
 
-<img src="https://raw.githubusercontent.com/source-foundry/fdiff/img/img/diff-example-crunch.png" width="600"/>
+<img src="https://raw.githubusercontent.com/source-foundry/fdiff/img/img/diff-example-crunch.png" width="450" style="display:block; margin-left: auto; margin-right: auto;"/>
 
 ## What it does