Add Documentation/module-signing.txt file

This patch adds the Documentation/module-signing.txt file that is
currently missing from the Documentation directory. The init/Kconfig
file references the Documentation/module-signing.txt file to explain
how kernel module signing works. This patch supplies this documentation.

Signed-off-by: James Solner <solner@alcatel-lucent.com>
Signed-off-by: David Howells <dhowells@redhat.com>