Mention lxc-checkconfig

This is important because the list below is not complete when one build one's own kernel.  Plus it is more robust in the face of changes.

docs/sources/installation/kernel.rst

@@ -115,6 +115,8 @@ Then run ``update-grub``, and reboot.
 Details
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+An easy way to check that the requirements below are met is to run `lxc-checkconfig`.
+
 Networking:
 
 - CONFIG_BRIDGE