doc: about.rst: use unicode emdash, use non-embedded URIs, add clarificaiton of when...

doc: about.rst: use unicode emdash, use non-embedded URIs, add clarificaiton of when a file gets its mutable-or-immutable nature
embedded URIs, although documented here:
http://docutils.sourceforge.net/docs/ref/rst/restructuredtext.html#embedded-uris
generate messages like this from rst2html --verbose:

quickstart.rst:3: (INFO/1) Duplicate explicit target name: "the tahoe-dev mailing list".

Also this patch prepends a "utf-8 BOM" to the beginning of the file.