Add draft of developer guide

docs/develop-guide.rst

+Developer Guide
+===============
+
+Document structures
+-------------------
+
+Return JSON of dbxref commands
+##############################
+
+dbxref returns json lists. Each entry in the list is a json object with at
+least the 'id' property. Additional properties should be the same for similar
+databases. For example most of the databases will contain a free text
+description of the entry which should be available via the 'description'
+property. Each database may have it's unique fields. These can be named as
+required.
+
+Resolve command
+###############
+
+Returns a list of json objects. Each object contains the id and locations.
+Locations is an object that provides the urls to the different file formats
+available for the entry. Valid formats are: 'html', 'xml', 'json', 'text'. If
+you need additional formats, feel free to add them. Each format is a property
+on the object and contains a list of all available urls. If a format is not
+available for an entry, it should not be present in the locations object.
+
+
+Handling of non existent dbxrefs
+###############################
+
+The proposed format for entries that cannot be found in the databases is as following:
+
+`
+[{'id': '<db>:<id>', "message": "no results found; probably invalid ID"}]
+`
+
+Handling of webservice errors
+#############################
+
+Try to be as precise as possible when an error occurs. The default (unprecise) response should be
+
+`
+[{'id': '<db>:<id>', "message": "Could not retrieve the requested entry due to problems."}]
+`
+
+You can include the original reponse:
+
+`
+[{'id': '<db>:<id>', "message": "Could not retrieve the requested entry due to problems.", "response" : "<original reponse>"}]
+`
+