Developer docs about existing subtrees.

Add some notes about the workflow for changes to code that exists in subtrees.

doc/developer-notes.md

@@ -408,6 +408,37 @@ GUI
     should not interact with the user. That's where View classes come in. The converse also
     holds: try to not directly access core data structures from Views.
 
+Subtrees
+----------
+
+Several parts of the repository are subtrees of software maintained elsewhere.
+
+Some of these are maintained by active developers of Bitcoin Core, in which case changes should probably go
+directly upstream without being PRed directly against the project.  They will be merged back in the next
+subtree merge.
+
+Others are external projects without a tight relationship with our project.  Changes to these should also
+be sent upstream but bugfixes may also be prudent to PR against Bitcoin Core so that they can be integrated
+quickly.  Cosmetic changes should be purely taken upstream.
+
+There is a tool in contrib/devtools/git-subtree-check.sh to check a subtree directory for consistency with
+its upstream repository.
+
+Current subtrees include:
+
+- src/leveldb
+  - Upstream at https://github.com/google/leveldb ; Maintained by Google, but open important PRs to Core to avoid delay
+
+- src/libsecp256k1
+  - Upstream at https://github.com/bitcoin-core/secp256k1/ ; actively maintaned by Core contributors.
+
+- src/crypto/ctaes
+  - Upstream at https://github.com/bitcoin-core/ctaes ; actively maintained by Core contributors.
+
+- src/univalue
+  - Upstream at https://github.com/jgarzik/univalue ; report important PRs to Core to avoid delay.
+
+
 Git and github tips
 ---------------------