Add pepschema directive in the documentation (#1092)

* Add `pepschema` directive docs

add links to eido and how to write a PEP schema guide

* add pep validation note

docs/snakefiles/configuration.rst

@@ -197,6 +197,12 @@ For those, one should always complement the pepfile with an ordinary :ref:`confi
 The rationale is that PEPs should be portable between different data analysis workflows (that could be applied to the same data) and even between workflow management systems.
 In other words, a PEP should describe everything needed about the data, while a workflow and its configuration should describe everything needed about the analysis that is applied to it.
 
+^^^^^^^^^^^^^^^
+Validating PEPs
+^^^^^^^^^^^^^^^
+
+Using the ``pepschema`` directive leads to an automatic parsing of the provided schema *and* PEP validation with the PEP validation tool -- `eido <http://eido.databio.org>`_. Eido schemas extend `JSON Schema <https://json-schema.org>`_ vocabulary to accommodate the powerful PEP features. Follow the `How to write a PEP schema <http://eido.databio.org/en/latest/writing-a-schema>`_ guide to learn more.
+
 .. _snakefiles-cluster_configuration:
 
 ----------------------------------