Create a README for XCMv1 detailing notable changes

xcm/src/v1/README.md

@@ -0,0 +1,24 @@
+# XCM Version 1
+
+## Changes to be aware of
+Most changes should automatically be resolved via the conversion traits (i.e. `TryFrom` and `From`). The list here is mostly for incompatible changes that result in an `Err(())` when attempting to convert XCM objects from v0.
+
+### Junction
+- `v0::Junction::Parent` cannot be converted to v1, because the way we represent parents in v1 has changed - instead of being a property of the junction, v1 MultiLocations now have an extra field representing the number of parents that the MultiLocation contains.
+
+### MultiLocation
+- The `try_from` conversion method will always canonicalize the v0 MultiLocation before attempting to do the proper conversion. Since canonicalization is not a fallible operation, we do not expect v0 MultiLocation to ever fail to be upgraded to v1.
+
+### MultiAsset
+- Stronger typing to differentiate between 1 class of `MultiAsset` and several classes of `MultiAssets` is introduced. As the name suggests, a `Vec<MultiAsset>` that is used on all APIs will instead be using a new type called `MultiAssets` (note the `s`).
+- All `MultiAsset` variants whose name contains "All" in it, namely `v0::MultiAsset::All`, `v0::MultiAsset::AllFungible`, `v0::MultiAsset::AllNonFungible`, `v0::MultiAsset::AllAbstractFungible`, `v0::MultiAsset::AllAbstractNonFungible`, `v0::MultiAsset::AllConcreteFungible` and `v0::MultiAsset::AllConcreteNonFungible`, will fail to be converted to v1 MultiAsset, since v1 does not contain these variants.
+- Similarly, all `MultiAsset` variants whose name contains "All" in it can be converted into a `WildMultiAsset`.
+- `v0::MultiAsset::None` is not represented at all in v1.
+
+### XCM
+- No special attention necessary
+
+### Order
+- `v1::Order::DepositAsset` and `v1::Order::DepositReserveAsset` both introduced a new `max_asset` field that limits the maximum classes of assets that can be deposited. During conversion from v0, the `max_asset` field is defaulted to 1.
+- v1 Orders that contain `MultiAsset` as argument(s) will need to explicitly specify the amount and dtails of assets. This is to prevent accidental misuse of `All` to possibly transfer, spend or otherwise perform unintended operations on `All` assets.
+- For v1 Orders that do allow the notion of `All` to be used as wildcards, they will instead use a new type called `MultiAssetFilter`.