Add document signatures chapter

docs/topics/index.rst

@@ -6,10 +6,13 @@ Introductions to all the key parts of Mayan EDMS you'll need to know:
 .. toctree::
    :maxdepth: 1
 
+   file_storage
+   initial_import
+   permissions
+   transformations
+   document_visualization
+   versioning
+   signatures
    indexes
    smart_links
-   permissions
-   document_visualization
    ocr
-   file_storage
-   transformations

docs/topics/signatures.rst

@@ -0,0 +1,24 @@
+===================
+Document signatures
+===================
+
+**Mayan EDMS** supports two types of document signatures, these are embedded and 
+detached signatures.  When a document with an embedded signature is 
+uploaded, this signature is readily detected as part of the document 
+inspection step.  If the public key corresponding to the signee of the 
+document is not found, **Mayan EDMS** will try to obtain it from the list of 
+keyserver specified in the config option :setting:`SIGNATURES_KEYSERVERS`.
+Failing that, **Mayan EDMS** will indicate that the document is signed
+but that it has no way to verify such signature.
+Existing non signed documents can be signed in one of two way:  
+by downloading the document, signing it, and uploading the signed document 
+as a new version of the existing one using **Mayan EDMS** :doc:`versioning support <versioning>`
+or by creating a detached signature for the non signed document and uploading 
+such detached signature file using the option likewise named menu option.
+
+Maintenance of the public keyring can be done using the ``Key management`` 
+functionality in the ``Setup menu`` 
+
+From this menu, key servers can be queried 
+and the results imported.  Public keys no longer needed can also be deleted 
+from this menu.