Update TODO file

TODO.rst

@@ -8,6 +8,28 @@
 Core
 ####
 
+New feature: sources
+====================
+
+Currently the way we get data into HSSG is a mess without any consistency. I
+propose the idea of "sources", "processors" and "sinks".
+
+- A source is some object which represents the initial point of data
+- A processor is some object which transforms data into other data
+- A sink is the final destination of the data which produces the actual output
+  file
+
+We already have artifacts as sinks and templates as filters. This only leaves
+sources to be implemented. A source is some object which implements the source
+protocol:
+
+- `GET-DATA`: Returns the data for further processing
+
+It is up to the source to implement for example some form of caching. A source
+could read data from a file, it could be hard-coded data, or it could use OCR
+to parse a message delivered by carrier pigeon.
+
+
 Cleanup
 =======