Add josh synchronization scripts

Author: Kobzol

README.md

@@ -16,3 +16,52 @@ This repository contains two main crates:
 
 The `std::simd` component now lives in the
 [`packed_simd_2`](https://github.com/rust-lang/packed_simd) crate.
+
+## Synchronizing josh subtree with rustc
+
+This repository is linked to `rust-lang/rust` as a [josh](https://josh-project.github.io/josh/intro.html) subtree. You can use the following commands to synchronize the subtree in both directions.
+
+You'll need to install `josh-proxy` locally via
+
+```
+cargo install josh-proxy --git https://github.com/josh-project/josh --tag r24.10.04
+```
+Older versions of `josh-proxy` may not round trip commits losslessly so it is important to install this exact version.
+
+### Pull changes from `rust-lang/rust` into this repository
+
+1) Checkout a new branch that will be used to create a PR into `rust-lang/stdarch`
+2) Run the pull command
+    ```
+    cargo run --manifest-path josh-sync/Cargo.toml rustc-pull
+    ```
+3) Push the branch to your fork and create a PR into `stdarch`
+
+### Push changes from this repository into `rust-lang/rust`
+
+NOTE: If you use Git protocol to push to your fork of `rust-lang/rust`,
+ensure that you have this entry in your Git config,
+else the 2 steps that follow would prompt for a username and password:
+
+```
+[url "git@github.com:"]
+insteadOf = "https://github.com/"
+```
+
+1) Run the push command to create a branch named `<branch-name>` in a `rustc` fork under the `<gh-username>` account
+    ```
+    cargo run --manifest-path josh-sync/Cargo.toml rustc-push <branch-name> <gh-username>
+    ```
+2) Create a PR from `<branch-name>` into `rust-lang/rust`
+
+#### Minimal git config
+
+For simplicity (ease of implementation purposes), the josh-sync script simply calls out to system git. This means that the git invocation may be influenced by global (or local) git configuration.
+
+You may observe "Nothing to pull" even if you *know* rustc-pull has something to pull if your global git config sets `fetch.prunetags = true` (and possibly other configurations may cause unexpected outcomes).
+
+To minimize the likelihood of this happening, you may wish to keep a separate *minimal* git config that *only* has `[user]` entries from global git config, then repoint system git to use the minimal git config instead. E.g.
+
+```
+GIT_CONFIG_GLOBAL=/path/to/minimal/gitconfig GIT_CONFIG_SYSTEM='' cargo run --manifest-path josh-sync/Cargo.toml -- rustc-pull
+```