Add timeliness example and warning

README.md

@@ -49,6 +49,26 @@ Some extensions may specify more specific rules for this field.
 
 This field superceedes the `sar:product_type` field.
 
+#### Timeliness
+
+Below you can find an example that shows how the timeliness fields could be used.
+
+The Copernicus programme releases products on three levels of timeliness:
+
+| Name                | Description                                                  | `product:timeliness`    | `product:timeliness_category` |
+| ------------------- | ------------------------------------------------------------ | ----------------------- | ----------------------------- |
+| Near Real-Time      | Delivered less than 3 hours after data acquisition.          | e.g. `PT3H` (3 hours)   | `NRT`                         |
+| Short Time-Critical | Delivered within 36 (Sentinel-6) to 48 (Sentinel-3) hours after data acquisition. | e.g. `PT36H` (36 hours) | `STC`                         |
+| Non Time-Critical   | Delivered typically within 1 month after data acquisition.   | e.g `P1M` (1 month)     | `NTC`                         |
+
+> \[!WARNING]
+>
+> Be careful when specifying the durations for `product:timeliness`. It is recommended to closely reflect the semantics of timeliness as specified by the provider. For example, if the timeliness is 36 hours, specify  `PT36H` instead of  `P1DT12H`, although allowed:
+>
+> > The standard does not prohibit date and time values in a duration  representation from exceeding their "carry over points". Thus, `PT36H` could be used as well as `P1DT12H` for representing the same duration. But keep in mind that `PT36H` is not the same as  `P1DT12H` when switching from or to Daylight saving time.
+>
+> Source: <https://en.wikipedia.org/wiki/ISO_8601#Durations>
+
 ## Contributing
 
 All contributions are subject to the