docs: add ARC-2 proposal

ARCs/ARC-2.md

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+# ARC-2: ITS Hub Register Existing ITS Tokens
+
+## Metadata
+
+- **ARC ID**: 2
+- **Author(s)**: CJ Cobb
+- **Status**: Draft
+- **Created**: 2024-12-3
+- **Last Updated**: 2024-12-3
+- **Target Implementation**: Q4 2024
+
+## Summary
+
+Add an admin/governance method on the ITS hub that allows registering a token that has already been deployed
+in p2p mode (without the hub). This allows existing ITS tokens that were deployed prior to the ITS hub release to 
+be registered with the hub and to be processed through the hub. At the edge contracts, these tokens will need to be
+migrated from p2p mode to hub mode.
+
+## Motivation
+
+### Background
+
+The ITS Hub tracks every deployed token, including the chains the token was deployed to, and will reject transfers
+of tokens to chains for which no deployment occurred. For example, if a token was deployed to chain A and B, a 
+transfer of that token from B to C will be rejected. However, tokens that were deployed prior to the launch of the
+ITS hub were deployed in p2p mode, and the ITS hub is not aware of those deployments. If one of those tokens is 
+then deployed to a hub connected chain, the ITS hub has an incomplete view of which chains the token is deployed 
+on, and will reject transfers that should succeed. For example, imagine token T is deployed on A and B in p2p 
+mode. The hub is not aware of either of these deployments. Then, token T is deployed from A to C via the hub. Now 
+the hub knows about the token T deployment on A and C, but not B. A transfer from C to B will then fail, even 
+though the token is deployed on B and thus the transfer should succeed.
+
+
+### Goals
+
+- Allow a token to be transferred to all chains where it is deployed
+- Maintain balance tracking and other invariants even when a token was not deployed via the hub
+
+## Design
+
+The ITS hub will expose an admin/governance method to register an existing token with the hub:
+
+```rust
+
+pub enum ExecuteMsg {
+    // all of the existing methods remain unchanged
+
+    #[permission(Elevated)]
+    RegisterToken {
+        deployed_chain: ChainNameRaw,
+        origin_chain: ChainNameRaw,
+        token_id: TokenId,
+        decimals: u8,
+        supply: Option<Uint256>, // None if not tracked for this chain
+        deployment_type: TokenDeploymentType // Trustless or Custom
+    }
+}
+```
+
+The above command will register the token and store all of the neccessary information. The supply is used to
+initialize balance tracking. The supply can be hard to specify with exact correctness, due to inflight transfers.
+Therefore, the command can be executed more than once with the same arguments albeit a different supply, which will
+overwrite the existing supply for that chain.
+
+The command will fail if the token instance already exists with a different decimals, origin_chain or
+deployment_type (or maybe we shouldn't fail, in case there is a mistake and we need to overwrite?).
+The command will fail if the supply is set to None and deployed_chain does not equal origin chain.
+The command will fail if either the deployed_chain or origin_chain is not registered with the hub.
+
+## Risks
+
+- For this to work effectively, there is operational overhead of determing every ITS token, and registering each
+with the ITS hub correctly. There is a possibility some are missed, or registered incorrectly, and thus need to be adjusted.
+
+### Changelog
+
+| Date | Revision | Author | Description |
+|------|-----------|---------|-------------|
+| 2024-12-3 | v1.0 | CJ Cobb | Initial ARC draft |