doc: Add crc32c subtree to developer notes

Author: laanwj

doc/developer-notes.md

@@ -858,6 +858,10 @@ Current subtrees include:
   - **Note**: Follow the instructions in [Upgrading LevelDB](#upgrading-leveldb) when
     merging upstream changes to the LevelDB subtree.
 
+- src/crc32c
+  - Used by leveldb for hardware acceleration of CRC32C checksums for data integrity.
+  - Upstream at https://github.com/google/crc32c ; Maintained by Google.
+
 - src/secp256k1
   - Upstream at https://github.com/bitcoin-core/secp256k1/ ; actively maintained by Core contributors.