Added small section on shiftfs to distro-compat doc.

Author: ctalledo

docs/distro-compat.md

@@ -29,3 +29,14 @@ package to do the upgrade as follows:
 ```console
 $ sudo apt-get update && sudo apt install --install-recommends linux-generic-hwe-18.04 -y
 ```
+
+### Using Sysbox On Kernels Without the Shiftfs Module
+
+Recent Ubuntu kernels carry a module called `shiftfs` that Sysbox uses as part
+of its container isolation strategy.
+
+However, some Ubuntu cloud images do not carry the module. In this case, Sysbox
+requires that Docker be configured in [userns-remap mode](https://docs.docker.com/engine/security/userns-remap/).
+
+The Sysbox installer will detect this condition and can automatically put
+Docker in userns-remap mode if desired. See [here](user-guide/install.md#docker-userns-remap) for details.

docs/user-guide/install.md

@@ -22,6 +22,8 @@ The Linux host on which Sysbox runs must meet the following requirements:
 
 ## Installing Sysbox
 
+**NOTE**: if you have a prior version of Sysbox already installed, please [uninstall it](#uninstalling-sysbox) first and then follow the installation instructions below.
+
 1) Download the latest Sysbox package from the [release](https://github.com/nestybox/sysbox-external/releases) page.
 
 2) Verify that the checksum of the downloaded file fully matches the expected/published one.
@@ -102,7 +104,7 @@ WARNING: No swap limit support
 
 ### Docker Userns-Remap
 
-In addition, the Sysbox installer will detect if Docker needs to be placed
+During installation, the Sysbox installer will also check if Docker needs to be placed
 in [userns-remap mode](https://docs.docker.com/engine/security/userns-remap/) or not.
 
 The installer uses the following logic: