Update README with version info

README.md

@@ -51,6 +51,30 @@ You can email <kontakt@marginalia.nu> with any questions or feedback.
 The bulk of the project is available with AGPL 3.0, with exceptions. Some parts are co-licensed under MIT, 
 third party code may have different licenses. See the appropriate readme.md / license.md.
 
+## Versioning
+
+The project uses modified Calendar Versioning, where the first two pairs of numbers are a year and month coinciding 
+with the latest crawling operation, and the third number is a patch number.
+
+```
+            version
+           --
+     yy.mm.VV
+     -----
+     crawl
+```
+
+For example, `23.03.02` is a release with crawl data from March 2023 (released in May 2023).
+It is the second patch for the 23.02 release.
+
+Versions with the same year and month are compatible with each other, or offer an upgrade path where the same 
+data set can be used, but across different crawl sets data format changes may be introduced, and you're generally
+expected to re-crawl the data from scratch as crawler data has shelf life approximately as long as the major release
+cycles of this project. After about 2-3 months it gets noticeably stale with many dead links.
+
+For development purposes, crawling is discouraged and sample data is available. See [📄&nbsp;run/readme.md](run/readme.md)
+for more information. 
+
 ## Funding
 
 ### Donations