rust: configure MSRV in Clippy
This setting makes Clippy never apply lints that are meant for
later versions. In case the target precise toolchain is the one
running, it does not make a difference, but this gives us a
machine-parseable specification that is pretty standard.
The README and `hg help rust` are updated to state that `clippy.toml`
is the single source of truth about that, also lifting a minor ambiguity:
it is fine if the MSRV is lagging behind the version in Debian testing.
Mercurial's documentation is kept in reStructuredText format, which is
a simple plain text format that's easy to read and edit:
http://docutils.sourceforge.net/rst.html
It's also convertible to a variety of other formats including standard
UNIX man page format and HTML. You'll need to install Docutils:
http://docutils.sourceforge.net/
Use the Makefile in this directory to generate the man and HTML pages.