docs: add 'color.diff.func' color to README

The color for hunk headers (`color.diff.func`) is hardcoded in the
script, and is not mentioned in the "Improved colors for the
highlighted bits" section of the README. Add it there so that function
headers with or without diff-so-fancy use the same color configuration.

Author: phil-blain

README.md

@@ -38,6 +38,7 @@ git config --global color.diff-highlight.newHighlight "green bold 22"
 
 git config --global color.diff.meta       "11"
 git config --global color.diff.frag       "magenta bold"
+git config --global color.diff.func       "146 bold"
 git config --global color.diff.commit     "yellow bold"
 git config --global color.diff.old        "red bold"
 git config --global color.diff.new        "green bold"