Merge pull request #2000 from prometheus/richih/exploratory_docs_2

Fix table

Author: RichiH

content/docs/introduction/design-doc.md

@@ -49,6 +49,8 @@ Sometimes we're looking even further into potential futures. The documents in
 this section are largely exploratory. They should be taken as informing our
 collective thoughts, not as anything concrete or specific.
 
-| Document | Initial date
-| [Prometheus is not feature complete](https://docs.google.com/document/d/1lEP7pGYM2-5GT9fAIDqrOecG86VRU8-1qAV8b6xZ29Q) | 2020-05
-| [Prometheus, OpenMetrics & OTLP](https://docs.google.com/document/d/1hn-u6WKLHxIsqYT1_u6eh94lyQeXrFaAouMshJcQFXs) | 2021-03
+| Document | Initial date |
+| -------- | ------------ |
+| [Prometheus is not feature complete](https://docs.google.com/document/d/1lEP7pGYM2-5GT9fAIDqrOecG86VRU8-1qAV8b6xZ29Q) | 2020-05 |
+| [Thoughts about timestamps and durations in PromQL]() | 2020-10 |
+| [Prometheus, OpenMetrics & OTLP](https://docs.google.com/document/d/1hn-u6WKLHxIsqYT1_u6eh94lyQeXrFaAouMshJcQFXs) | 2021-03 |