docs(mimic-iv-ed): link README to TIMING_NOTES

Surface the #1910 caveat from the module entrypoint.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: Chessing234

mimic-iv-ed/README.md

@@ -1,3 +1,7 @@
 # MIMIC-IV-ED
 
-MIMIC-IV-ED is a publicly available database of emergency department visit data. You can read more about the dataset on [the PhysioNet project page](https://mimic.mit.edu/docs/iv/modules/ed/).
+MIMIC-IV-ED is a publicly available database of emergency department visit data. You can read more about the dataset on [the PhysioNet project page](https://mimic.mit.edu/docs/iv/modules/ed/).
+
+## Timing caveats
+
+`edstays.intime` is an administrative ED registration time. Pyxis and prescription timestamps can legitimately fall slightly before that window — see [docs/TIMING_NOTES.md](docs/TIMING_NOTES.md).