Update the README.
Update the README.

- remove obsolete references to rpm, debian, macos builds
- put the tarball build instructions before the svn ones.
- more markup-ish headers
- misc other cleanup


svn path=/trunk/ogg/; revision=18048

nicolaichuk/ogg,xiph/ogg,felipebetancur/ogg,nicolaichuk/ogg,ShiftMediaProject/ogg,youfearm3/ogg,brion/ogg,emscripten-ports/Ogg,ksophocleous/ogg,ksophocleous/ogg,youfearm3/ogg,eidy/libogg,mwgoldsmith/ogg,eidy/libogg,brion/ogg,xiph/ogg,felipebetancur/ogg,emscripten-ports/Ogg,ShiftMediaProject/ogg,mwgoldsmith/ogg