[doc] add info about the iwyu target
[doc] add info about the iwyu target

Added information about the cmake-generated target to run IWYU
against the updated files.

In addition, fixed the following error reported by asciidoctor:
  asciidoctor: ERROR: README.adoc: line 414: only book doctypes \
    can contain level 0 sections

Change-Id: I87da493486500cde0cd226614f8a19985d295a96
Reviewed-on: http://gerrit.cloudera.org:8080/7928
Reviewed-by: Adar Dembo <866ca730bfc7f544e3ebce604ceeba4879f23df0@cloudera.com>
Reviewed-by: Mike Percy <e07a1fd797b473a9c849e618aa2ce06d8055760b@apache.org>
Tested-by: Mike Percy <e07a1fd797b473a9c849e618aa2ce06d8055760b@apache.org>

helifu/kudu,cloudera/kudu,helifu/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,andrwng/kudu,EvilMcJerkface/kudu,andrwng/kudu,helifu/kudu,andrwng/kudu,cloudera/kudu,EvilMcJerkface/kudu,helifu/kudu,andrwng/kudu,cloudera/kudu,InspurUSA/kudu,InspurUSA/kudu,InspurUSA/kudu,InspurUSA/kudu,helifu/kudu,cloudera/kudu,helifu/kudu,helifu/kudu,andrwng/kudu,EvilMcJerkface/kudu,andrwng/kudu,EvilMcJerkface/kudu,helifu/kudu,cloudera/kudu,cloudera/kudu,EvilMcJerkface/kudu,cloudera/kudu,InspurUSA/kudu,InspurUSA/kudu,helifu/kudu,andrwng/kudu,andrwng/kudu,InspurUSA/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,cloudera/kudu,InspurUSA/kudu,cloudera/kudu,andrwng/kudu,EvilMcJerkface/kudu