fix readme logo for dark mode (#160)
fix readme logo for dark mode (#160)

The previous version of the logo with the project name was not readable
in Github's dark mode due to a lack of contrast. This modification adds
a background to the text itself so that it is readable regardless of the
the display background color.
goatshriek/stumpless,goatshriek/stumpless,goatshriek/stumpless,goatshriek/stumpless