Merge pull request #1 from deeplusplus/deeplusplus-patch-3

Update README.md

Author: deeplusplus

README.md

@@ -100,7 +100,7 @@ If you're replicating SQL databases, whether MySQL, Postgres, or SQL Server, [ch
 ## Level 6
 
 - Concepts
-  - database cracking: *explanation needed*
+  - database cracking: https://stratos.seas.harvard.edu/files/IKM_CIDR07.pdf
   - learned indexes: *explanation needed*
   - XTID exhaustion: *explanation needed*
   - Worst Case Optimal Join: *explanation needed*