Add overview of Pungi to documentation

Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>

doc/about.rst

@@ -2,7 +2,7 @@
  About Pungi
 =============
 
-Pungi is a distribution compose tool.
+*Pungi* is a distribution compose tool.
 
 Composes are release snapshots that contain release deliverables such as:
 
@@ -19,6 +19,24 @@ Composes are release snapshots that contain release deliverables such as:
   - images for PXE boot
 
 
+Tool overview
+=============
+
+*Pungi* consists of multiple separate executables backed by a common library.
+
+The main entry-point is the ``pungi-koji`` script. It loads the compose
+configuration and kicks off the process. Composing itself is done in phases.
+Each phase is responsible for generating some artifacts on disk and updating
+the ``compose`` object that is threaded through all the phases.
+
+*Pungi* itself does not actually do that much. Most of the actual work is
+delegated to separate executables. *Pungi* just makes sure that all the
+commands are invoked in the appropriate order and with correct arguments. It
+also moves the artifacts to correct locations.
+
+
 Links
 =====
 - Upstream GIT: https://pagure.io/pungi/
+- Issue tracker: https://pagure.io/pungi/issues
+- Questions can be asked on *#fedora-releng* IRC channel on FreeNode