Modernize README using Markdown

This commit also:

  * adds a project description at the top
  * simplifies the steps to build/link a user
    application with LTTng-UST and provides examples
    of commands
  * updates the Package contents list

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>