docs: Use relative links in install README

Simplify the installation README by using relative URLs - let github
expand them automatically for readers.

Signed-off-by: James O. D. Hunt <james.o.hunt@intel.com>

install/README.md

@@ -5,13 +5,13 @@ See the [hardware requirements](https://github.com/kata-containers/runtime/blob/
 
 Select your preferred distribution:
 
-* [CentOS](https://github.com/kata-containers/documentation/blob/master/install/centos-installation-guide.md)
-* [Fedora](https://github.com/kata-containers/documentation/blob/master/install/fedora-installation-guide.md)
-* [Red Hat](https://github.com/kata-containers/documentation/blob/master/install/rhel-installation-guide.md)
-* [Ubuntu](https://github.com/kata-containers/documentation/blob/master/install/ubuntu-installation-guide.md) 
+* [CentOS](centos-installation-guide.md)
+* [Fedora](fedora-installation-guide.md)
+* [Red Hat](rhel-installation-guide.md)
+* [Ubuntu](ubuntu-installation-guide.md)
 
 For further information, see:
 
 * The [the upgrading document](../Upgrading.md).
-* The [developer guide](https://github.com/kata-containers/documentation/blob/master/Developer-Guide.md).
+* The [developer guide](../Developer-Guide.md).
 * The [runtime documentation](https://github.com/kata-containers/runtime/blob/master/README.md).