Improve README.md

Improve README.md

Changes:

* Harmonize voice and style.

* Add non-breaking spaces and hyphens where missing.

* Add many internal and external links.

* Add `lttng-crash` and the Python bindings to the list of components.

* Use `≥` instead of `>=` (welcome to 2020).

* Do not specify why you need to configure with `--disable-epoll` to use
  a Linux kernel < 2.6.27: I don't think the end user needs to know
  about poll() vs. epoll().

* Do not mention Mathieu Desnoyers and Paul E. McKenney (this is a
  README, not the `THANKS` file).

* Mention Babeltrace 2 instead of Babeltrace.

* Use `https://babeltrace.org/` instead of
  `https://lttng.org/babeltrace`.

* Add kmod to the list of optional dependencies.

* Specify that you can use `--enable-embedded-help` at build
  configuration time if you don't plan to have a manual pager installed
  on your system.

* Don't mention the known GNU gold bug
  (`http://sourceware.org/bugzilla/show_bug.cgi?id=11317`).

  Again, the end user only cares about which minimal version of the tool
  to use, not why.

* Use clear, ordered build steps.

  For the configuration step, specify which options are available and
  what they change.

* Remove the link to `doc/quickstart.txt`.

  Link to the LTTng Documentation and online manual pages instead, which
  are more complete.

* Remove the link to `doc/streaming-howto.txt`: the LTTng Documentation
  and online manual pages cover this topic.

* Change the "Contact" section for the "Community" section and put all
  the community/communication links in there.

* Remove the "Package contents" section: end users do not care about
  this and we're not keeping it updated as we amend the project's source
  anyway.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I4e4c0b183d9a18243db23cde3edb608e32c5e30e
(cherry picked from commit f8bd3a12204c10743dfbc456d363e30894eddfc5)
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>