Added time-keeping and kernel downloading

readme.md

@@ -76,6 +76,24 @@ are measured here rather than cited.
 | Full-force LEO, 1 day | not yet measured | — | — |
 | Ensemble of N states, scaling vs. cores | not yet measured | — | — |
 
+## Kernels
+
+Ephemerides and Earth orientation come from NAIF kernels, which are too large to
+commit. `data/kernels.tsv` pins each one by URL and SHA-256:
+
+```sh
+scripts/fetch-kernels.sh            # into data/kernels
+scripts/fetch-kernels.sh /somewhere # or wherever you keep them
+```
+
+The script verifies checksums and leaves an already-correct file alone. Point
+`ASTROKIT_KERNELS` at the directory if it is not `data/kernels`. Tests that need
+kernels say so and skip when they are absent.
+
+`earth_latest_high_prec.bpc` is deliberately not used: a result that cites "the
+latest Earth orientation kernel" cannot be reproduced, so the dated snapshot
+covering the same span is pinned instead.
+
 ## Development
 
 Enable the pre-commit hook once per clone: