Merge the generic bits of the release notes into the readme.

Also switch to markdown-style headings to the benefit of github.

README.txt

-== Opusfile ==
+## Opusfile ##
 
-Library for decoding .opus files, including seeking support.
+The opusfile and opusurl libraries provide a high-level API for
+decoding and seeking within .opus files on disk or over http(s).
+
+opusfile depends on libopus and libogg.
+opusurl depends on opusfile and openssl.
+
+The library is functional, but there are likely issues
+we didn't find in our own testing. Please give feedback
+in #opus on irc.freenode.net or at opus@xiph.org.
+
+Programming documentation is available in tree and online at
+https://opus-codec.org/docs/