Reorganize "Installing" section slightly to put "git building" bit first.

Originally at the time of editing, the "Installation is done with the ..." command blurb comes *before* the notice about if you're building from git.  I believe it is more sane to have that come first in the README section for installing, as it will be seen before the actual installation commands are, so people will first see that if they are building from git they'll need to run `./autogen.sh`.

While this is a minor change, it is still, I believe, a logical change.
@DarthGandalf The "If" block ends is the git-related data.  We could probably prepend that sentence with "NOTE: " to make a note that you'll need additional software if compiling from git.

Currently, the "Installation is done..." and "If you are building from git..." sections are switched in position, so the git stuff comes after "Installation is done with..."