To use Font-Awesome icons font rather than PNG images, build your AsciiDoc source files with attribute iconsfont - use -a iconsfont=font-awesome

Admonition NOTE block

NOTE: This is an example note.

[NOTE]
Alternatively, you can specify the paragraph admonition style explicitly using an AttributeList element.

Renders:

This is an example note.
Alternatively, you can specify the paragraph admonition style explicitly using an AttributeList element.

Admonition IMPORTANT block

IMPORTANT: This is another example more important.

Renders:

This is another example more important.

Admonition CAUTION block

CAUTION: This is another example important but less.

Renders:

This is another example important but less.

Admonition WARNING block

WARNING: This is a warning example.

Renders:

This is a warning example.

Admonition NOTE custom icon block

[icon="./images/icons/example.png"]
NOTE: What lovely war.

Renders:

What lovely war.

Admonition TIP block

[TIP]
.A TIP admonition block
=====================================================================
Qui in magna commodo, est labitur dolorum an. Est ne magna primis
adolescens.

. Fusce euismod commodo velit.
. Vivamus fringilla mi eu lacus.
  .. Fusce euismod commodo velit.
  .. Vivamus fringilla mi eu lacus.
. Donec eget arcu bibendum
  nunc consequat lobortis.
=====================================================================

Renders:

A TIP admonition block

Qui in magna commodo, est labitur dolorum an. Est ne magna primis adolescens.

  1. Fusce euismod commodo velit.
  2. Vivamus fringilla mi eu lacus.
    1. Fusce euismod commodo velit.
    2. Vivamus fringilla mi eu lacus.
  3. Donec eget arcu bibendum nunc consequat lobortis.

Learn More

Behind the scene

Have a look in the LESS source file less/asciidoc-bootstrap.less, you should have at least this line activated
[https://raw.github.com/llaville/asciidoc-bootstrap-backend/master/less/asciidoc/admonition-iconsfont.less]
 :

@import "asciidoc/admonition-iconsfont.less";