Add 'light = false' w/ comment in gitconfig (#1073)

* Add 'light = false' w/ comment in gitconfig

As mentioned in #1072, the macOS default terminal is light and it's challenging to find documentation on how to make delta copacetic with it, so this adds an explicit `light = false` in the `.gitconfig` example, with a comment explaining that it can be changed for light terminals.

* Update README.md

fix indentation

README.md

@@ -26,6 +26,7 @@
 
 [delta]
     navigate = true  # use n and N to move between diff sections
+    light = false    # set to true if you're in a terminal w/ a light background color (e.g. the default macOS terminal)
 
 [merge]
     conflictstyle = diff3