docs: Add detail to install README
Add a sentence introducing the install guides in the installation
README.
# Kata Containers installation user guides
+Select your preferred distribution:
+
* [Fedora](https://github.com/kata-containers/documentation/blob/master/install/fedora-installation-guide.md)
* [Ubuntu](https://github.com/kata-containers/documentation/blob/master/install/ubuntu-installation-guide.md)
