Add instructions

Author: oli-obk

README.md

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+This crate is regularly synced with its mirror in the rustc repo at `compiler/rustc_smir`.
+
+We use `git subtree` for this to preserve commits and allow the rustc repo to
+edit these crates without having to touch this repo. This keeps the crates compiling
+while allowing us to independently work on them here. The effort of keeping them in
+sync is pushed entirely onto us, without affecting rustc workflows negatively.
+This may change in the future, but changes to policy should only be done via a
+compiler team MCP.
+
+## Instructions for syncing
+
+### Updating this repository
+
+In the rustc repo, execute
+
+```
+git subtree push --prefix=compiler/rustc_smir url_to_your_fork_of_project_stable_mir some_feature_branch
+```
+
+and then open a PR of your `some_feature_branch` against https://github.com/rust-lang/project-stable-mir
+
+### Updating the rustc librar
+
+
+In the rustc repo, execute
+
+```
+git subtree pull --prefix=compiler/rustc_smir https://github.com/rust-lang/project-stable-mir smir
+```
+
+Note: only ever sync to rustc from the project-stable-mir's `smir` branch. Do not sync with your own forks.
+
+Then open a PR against rustc just like a regular PR.