Add git-subtree-check.sh script

contrib/devtools/README.md

@@ -81,3 +81,16 @@ It will do the following automatically:
 
 See doc/translation-process.md for more information.
 
+git-subtree-check.sh
+====================
+
+Run this script from the root of the repository to verify that a subtree matches the contents of
+the commit it claims to have been updated to.
+
+To use, make sure that you have fetched the upstream repository branch in which the subtree is
+maintained:
+* for src/secp256k1: https://github.com/bitcoin/secp256k1.git (branch master)
+* for sec/leveldb: https://github.com/bitcoin/leveldb.git (branch bitcoin-fork)
+
+Usage: git-subtree-check.sh DIR COMMIT
+COMMIT may be omitted, in which case HEAD is used.