In README.md replace reference to "stylish" brower extension with "stylus" because of malware issues

[jeanmonet]

It appears that the "stylish" browser extension is now considered to be spyware. See:

• https://meta.stackoverflow.com/a/294045/11750716 (end of post)
• https://github.com/openstyles/stylus/wiki/FAQ#what-are-the-main-differences-and-improvements-over-the-original-stylish-add-on

Please replace with a reference to "stylus" browser extension.