Update project's README

This patch rewrites the README file as `README.adoc`.

`README.adoc` indicates:

* What Babeltrace 2 is.
* How Babeltrace 2 is different from Babeltrace 1.
* What are the project's build-time and run-time requirements.
* How to build Babeltrace 2.
* How to build Babeltrace 2 for development.
* Where the Babeltrace community is.

The file refers to the Babeltrace website for the reader to learn more
about the project.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I1b7ce8e949fb91616f2672f94b7e8d346a6a368b
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2177