docs: add simplicity code update instructions

Author: delta1

doc/simplicity-c-code-update.md

@@ -0,0 +1,39 @@
+# Updating Simplicity C code in Elements
+
+This document describes how to update the Simplicity C code subtree in the Elements repository.
+
+Reference: https://github.com/BlockstreamResearch/simplicity/issues/329#issuecomment-3715844042
+
+## Simplicity
+
+- Clone or pull the latest `master` branch of the [Simplicity](https://github.com/BlockstreamResearch/simplicity) repository.
+
+- Split out a new subtree of the Simplicity `C` source to the `C-master` branch.
+
+```
+git subtree split -P C  -b C-master
+```
+
+- Push the `C-master` branch to a public remote.
+
+```
+git push <remote> C-master
+```
+
+## Elements
+
+- Add a reference to the above simplicity remote in your Elements repo.
+
+```
+git remote add <new-remote-name> git@github.com:<user>/simplicity.git
+```
+
+- Pull and squash the `C-master` branch subtree into the `src/simplicity` directory in Elements.
+
+```
+git subtree pull --prefix src/simplicity <new-remote-name> C-master --squash
+```
+
+- Run build and tests.
+
+- Create a new PR to Elements.