Simplify Linux README for new users.

After trying to install Breach on Xubuntu 14.04 and running into both of
these issues, I'm suggesting this rewrite for the README in the hopes that
it can help new users get through the process more easily.

The salient changes are:

* Add a bit of preamble to orient a new user, informing them that there are
  two common problems
* Reorder the issues to present them in the order they occur
* Directly offer specific solutions for libudev.so.0, instead of linking to
  an outside page
* Add a second suggested chrome-sandbox path.