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.

The initial version of this patch provided old documentation
that was a mixture of the old RHEL style GPG signing.

Version 1: Updated the documentation to described the current
           implementation using x509 certificate signing.

Version 2: fixes grammar/spelling mistakes and removes
           trailing whitespaces.

Version 3, fixes grammar/spelling mistakes.

Version 4: Include updates from David Howells and fixes for
           spelling mistakes.

Signed-off-by: James Solner <solner@alcatel-lucent.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>