chore: add some docs on syncing noir (#6340)

This PR adds some quick instructions on how to sync between the two
repositories to make it easier for other people to perform syncs
themselves.

noir/README.md

@@ -2,3 +2,32 @@
 
 We subrepo noir into the folder `noir-repo`.
 This folder contains dockerfiles and scripts for performing our custom build of noir for the monorepo.
+
+# Syncing with the main Noir repository
+
+In order to keep aztec-packages in step with the main Noir repository we need to periodically sync between them.
+
+Syncing from aztec-packages into noir currently attempts to revert any changes in Noir since the last sync so it's recommended to always sync from Noir first to ensure that aztec-packages is up-to-date.
+
+## Syncing from Noir to aztec-packages.
+
+To start the sync run [this action](https://github.com/AztecProtocol/aztec-packages/actions/workflows/pull-noir.yml) manually (click the "Run Workflow" button in the top right). aztec-bot will then open a new PR which does the initial sync, this will have merge conflicts with master which will need to be resolved.
+
+Most of these will be due to simultaneous development in the two repositories but there are a few cases which are due to the sync process: 
+1. Replace the dependency on `@aztec/bb.js` in `noir-lang/noir_js_backend_barretenberg` to use the version built in this repository:
+  a. To do this, search for instances of `"@aztec/bb.js":` within package.json files and replacing the versions with `"portal:../../../../barretenberg/ts"` (number of directories to go up by may vary)
+2. Run `yarn install` in `noir/noir-repo` in order to update `yarn.lock`.
+3. Run a search and replace on `require_command wasm-opt` to `#require_command wasm-opt`
+
+We need to do this as `noir-lang/noir` uses a fixed version of barretenberg but in aztec-packages we test against the version of barretenberg built from the same commit. 
+
+## Syncing from aztec-packages to Noir.
+
+When syncing from aztec-packages to Noir it's important to check that the latest release of `bb` uses the same ACIR serialization format as the current master commit. This is because Noir uses a released version of barretenberg rather than being developed in sync with it, it's then not possible to sync if there's been serialization changes since the last release.
+
+To start the sync run [this action](https://github.com/AztecProtocol/aztec-packages/actions/workflows/mirror-noir-subrepo.yml) manually (click the "Run Workflow" button in the top right). aztec-bot will then open a new PR in the `noir-lang/noir` repository which does the initial sync, this will have merge conflicts with master which will need to be resolved.
+
+Most of these will be due to simultaneous development in the two repositories but there are a few cases which are due to the sync process: 
+1. Replace the dependency on `@aztec/bb.js` in `noir-lang/noir_js_backend_barretenberg` to use the latest `aztec-packages` release version
+2. Run `yarn install` in order to update `yarn.lock`.
+3. Run a search and replace on `#require_command wasm-opt` to `require_command wasm-opt`