Make room for Dockerfiles for multiple operating system versions

It would be good to have Dockerfiles for multiple Fedora versions, and
possibly even other operating systems like Red Hat Enterprise Linux or
CentOS, so that people can use the toolbox on different host OSes.

In the future, it would be nice to have multiple toolbox containers
corresponding to different OSes exist side by side. One could then
have a Fedora 29 environment on a Fedora 28 host and so on.

README.md

@@ -18,8 +18,12 @@ seamlessly integrates with the rest of the operating system.
 ## Usage
 
 ### Create the basic Fedora Toolbox image:
+Use one of the sub-directories under `images` to create a `fedora-toolbox`
+image for a Fedora version that matches your host. eg., let's say we have a
+Fedora 28 host.
 ```
-[user@hostname fedora-toolbox]$ buildah bud --tag fedora-toolbox:28 .
+[user@hostname fedora-toolbox]$ cd images/fedora/f28
+[user@hostname f28]$ buildah bud --tag fedora-toolbox:28 images/fedora/f28
 STEP 1: FROM docker://registry.fedoraproject.org/fedora:28
 Getting image source signatures
 …
@@ -27,9 +31,9 @@ Getting image source signatures
 …
 [user@hostname fedora-toolbox]$
 ```
-Modify the Dockerfile to match your taste and Fedora version. The image should
-be tagged as `fedora-toolbox` with a suffix matching the host Fedora version.
-eg., `fedora-toolbox:29`, etc..
+Note that the image should be tagged as `fedora-toolbox` with a suffix
+matching the host Fedora version. In this example, it should be
+`fedora-toolbox:28`.
 
 ### Create your Fedora Toolbox container:
 ```

images/fedora/f28/README.md

@@ -0,0 +1 @@
+../../../README.md