Update documentation

Add screenshot

README.md

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 *musescore-serifjazz-chords* provides a [Musescore](https://musescore.org/) [chord symbol style file](https://musescore.org/en/handbook/chord-symbols#appearance) with Jazz-style superscripts and font distinctions, while using standard (sans-)serif fonts congruent with standard engraving formatting.
 
+![Screenshot](https://yingtongli.me/git/musescore-serifjazz-chords/plain/Screenshot.png)
+
 ## Usage
 
 From the [files in this repository](https://yingtongli.me/git/musescore-serifjazz-chords/tree/), download the XML file corresponding to the desired staff spacing. XML files are available for the default spacing of 1.764mm, and for various Rastral sizes.
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 To achieve better kerning with A chords, use `*`. For example, type `A*7` or `A*o` instead of `A7` or `Ao` to position the modifier/extension more snugly with the ‘A’. The exception is the half diminished (ø) chord, for which you should type `A*h` (not `A*0`).
 
+### Fonts
+
+The layout parameters in these XML files are designed for, and expect, the *FreeSerif* and *FreeSans* fonts.
+
 ## Information for contributors
 
 XML files corresponding to specific staff sizes should not be directly edited. They are automatically generated from *chords_serifjazz.m4.xml*. This is an [m4](https://en.wikipedia.org/wiki/M4_%28computer_language%29) file, which automatically substitutes appropriate layout parameters for each staff size.