Mention two more bug fixes in README.txt

Updates to README.txt by Bill Wharrie

Updated sl.po -- Slovenian submitted by Martin Srebotnjak

Updated sl.po -- Slovenian submitted by Martin Srebotnjak

Fix build of .mo files on all platforms

Make append-recording insert clip boundaries again

remove incorrect information about PortMidi

Flush .aup before writing .au; no redundant out-of-space messages

Make the (en) welcome window say it's a Beta test version, not alpha...

... This gets in too late for the translation files, but very few of them
included a translation for this message.  So, it will appear English for all.

No longer force local FFMPEG and LIBFLAC