Update README.txt
Add osd_subtitle.xml

Adds a non-default definition for text subtitles
* font based on basesmall, with size=30, yellow with black outline
Use Ubuntu font in htmls/progdetails_page*.html

* use the Ubuntu font in the 'Programme Details' screens, rather than the default serif font
  (press i with a recording highlighted to see this screen)
* adjust the font sizes down a little to match the rest of the theme