Documentation for the export subcommand

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+Moving Data between Collections
+===============================
+
+Entries can be exported from a lesana collection to another using the
+``lesana export`` command and a jinja2 template.
+
+The template should generate a yaml file that is a valid lesana entry
+for the destination collection and it can use the fields of the starting
+collection as variables. The variable ``entry`` is also available and
+gives complete access to the entry of the original collection, so fields
+with names that aren't valid jinja templates can be accessed as
+``entry.data[<field-name>]``.
+
+E.g. to convert between a collection with fields ``name``,
+``short-desc``, ``desc`` to a collection with fields ``name``,
+``description`` one could use the following template::
+
+   name: {{ name }}
+   description: |
+       {{ entry.data.[short-desc] }}
+
+       {{ desc | indent(width=4, first=False) }}
+
+From the origin collection you can then run the command::
+
+   lesana export <path/to/destination/collection> <path/to/template>
+
+to export all entries.
+
+You can also export just a subset of entries by adding a xapian query
+with the parameter ``--query``; you can test the search using::
+
+   lesana search --all <some search terms>
+
+and then actually run the export with::
+
+   lesana search --query '<some search terms>' <destination> <template>
+
+note that in this second command the spaces in the search query have to
+be protected from the shell.