Update distribution support information

Update the documentation with regards to our supported
distribution statement. This Fixes #1108

doc/source/building.rst

@@ -61,16 +61,39 @@ Building Images
 Supported Distributions
 -----------------------
 
-KIWI can build the above image types for distributions
-which are **equal** or **newer** compared to the following list:
-
-*  CentOS 7
-*  Fedora 25
-*  openSUSE Leap 42
-*  Red Hat Enterprise 7
-*  SUSE Linux Enterprise 12
-*  openSUSE Tumbleweed
-*  Ubuntu 16.04 (Xenial Xerus)
-
-For anything older please consider to use the legacy KIWI version *v7.x*
-For more details on the legacy kiwi, see: :ref:`legacy_kiwi`.
+KIWI can build images for the distributions which are **equal** or **newer** compared to
+the table below. For anything older please consider to use the legacy KIWI version *v7.x*
+For more details on the legacy kiwi, see: :ref:`legacy_kiwi`
+
+The most compatible environment is provided if the build host is of the same distribution
+than the target image. For images build with the Open Build Service (OBS) this always
+applies. In any other case please check the table if your *Host* vs. *Image* target
+combination is known to be supported.
+
++------------------+------------+---------------+------------------+----------+-------------+-----------+
+| Host / Image     | CentOS 7   | Fedora 25     | openSUSE Leap 15 | RHEL 7   | openSUSE TW | Ubuntu 16 |
++==================+============+===============+==================+==========+=============+===========+
+| CentOS 7         | yes        | no            | no               | yes      | no          | no        |
++------------------+------------+---------------+------------------+----------+-------------+-----------+
+| Fedora 25        | untested   | yes           | no               | untested | no          | no        |
++------------------+------------+---------------+------------------+----------+-------------+-----------+
+| openSUSE Leap 15 | untested   | **note:dnf**  | yes              | untested | no          | no        |
++------------------+------------+---------------+------------------+----------+-------------+-----------+
+| RHEL 7           | untested   | no            | no               | yes      | no          | no        |
++------------------+------------+---------------+------------------+----------+-------------+-----------+
+| openSUSE TW      | untested   | **note:dnf**  | yes              | untested | yes         | no        |
++------------------+------------+---------------+------------------+----------+-------------+-----------+
+| Ubuntu 16        | no         | no            | no               | no       | no          | yes       |
++------------------+------------+---------------+------------------+----------+-------------+-----------+
+
+.. note::
+
+   **dnf** is the package manager used on Fedora and RHEL and is the successor
+   of yum. When kiwi builds images for this distributions the latest version
+   of dnf is required to be installed on the host to build the image
+
+In general our goal is to support any major distribution with KIWI. However for building
+images we rely on core tools which are not under our control. Also several design
+aspects of distributions are different and not influenced by us. There are many side
+effects that can be annoying especially if the building host is not of the same distribution
+vendor than the image target.