Document shortcomings with rootless podman

Signed-off-by: Daniel J Walsh <dwalsh@redhat.com>

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+# Shortcomings of Rootless Podman
+
+The following list categorizes the known issues and irregularities with running Podman as a non-root user.  Although currently functional, there is still a number of work items that are under consideration to be added.  These proposed changes are in varying degrees of design and development.
+
+Contributors are more than welcomed to help with this work.  If you decide to carve off a piece and work on it, please create an issue in [GitHub](https://github.com/containers/libpod/issues), and assign it to yourself.  If you find other unexpected behaviour with rootless Podman and feel it’s warranted, please feel free to update this document.
+
+* Podman can not create containers that bind to ports < 1024.
+  * The kernel does not allow processes without CAP_NET_BIND_SERVICE to bind to low ports.
+* Lacking “How To” documentation or documentation in general
+* If /etc/subuid and /etc/subgid not setup for a user, then podman commands
+can easily fail
+  * This can be a big issue on machines using Network Based Password information (FreeIPA, Active Directory, LDAP)
+  * We are working to get support for NSSWITCH on the /etc/subuid and /etc/subgid files.
+* No cgroup Support (hopefully fixed when cgroups V2 happens).
+  * Cgroups V1 does not safely support cgroup delegation.
+  * Cgroups V2 development for container support is ongoing.
+* Can not share container images with CRI-O or other users
+* Difficult to use additional stores for sharing content
+* Does not work on NFS homedirs
+  * NFS enforces file creation on different UIDs on the server side and does not understand User Namespace.
+  * When a container root process like YUM attempts to create a file owned by a different UID, NFS Server denies the creation.
+* Does not work with homedirs mounted with noexec/nodev
+  * User can setup storage to point to other directories they can write to that are not mounted noexec/nodev
+* Can not use overlayfs driver, but does support fuse-overlayfs
+  * Ubuntu supports non root overlay, but no other Linux distros do.
+* Only other supported driver is VFS.
+* No KATA Container support
+* No CNI Support
+  * CNI wants to modify IPTables, plus other network manipulation that I requires CAP_SYS_ADMIN.
+  * There is potential we could probably do some sort of blacklisting of the relevant plugins, and add a new plugin for rootless networking - slirp4netns as one example and there may be others
+* Cannot use ping
+  * [(Can be fixed by setting sysctl on host)](https://github.com/containers/libpod/blob/master/troubleshooting.md#5-rootless-containers-cannot-ping-hosts)
+* Requires new shadow-utils (not found in older (RHEL7/Centos7 distros) Should be fixed in RHEL7.7 release
+* A few commands do not work.
+  * mount/unmount (on fuse-overlay)
+     * Only works if you enter the mount namespace with a tool like buildah unshare
+  * podman stats (Lack of Cgroup support)
+  * Checkpoint and Restore (CRIU requires root)
+  * Pause and Unpause (no freezer cgroup)
+* Issues with higher UIDs can cause builds to fail
+  * If a build is attempting to use a UID that is not mapped into the user namespace mapping for a container, then builds will not be able to put the UID in an image.