Add signature

[jferard]

There is a "META-INF/documentsignatures.xml" file.

See:

• file:///home/jferard/Documents/Documentations/OpenDocument-v1.2-os/OpenDocument-v1.2-os-part3.html#Digital_Signature_File
• https://www.w3.org/TR/2008/REC-xmldsig-core-20080610/#sec-Signature

[jferard]

See:

• https://github.com/bcgit/bc-java/tree/master/pg/src/main/java/org/bouncycastle/openpgp/examples