docs: Update README with a deprecation notice (if we decide to deprecate)

[andrew-ifrita]

Related to paritytech/asset-transfer-api#652.

Staging as a draft for now so this is ready if/when asset-transfer-api is deprecated.

Quick link to rendered README

[acatangiu]

while the asset-transfer-api might be deprecated, the asset-transfer-api-registry should still be kept alive and well maintained

afaik there is no alternative registry available in the ecosystem with same/equivalent level of information

[acatangiu]

I asked @franciscoaguirre to check with Paraspell and Nova (and others) where they get their information from. If there is no ecosystem high-quality alternative, we need to own this within Parity and keep it alive and well maintained (up-to-date)

[franciscoaguirre]

This is the assets registry used by paraspell: https://github.com/paraspell/xcm-tools/blob/main/packages/assets/src/maps/assets.json

It can be used as a standalone package: https://www.npmjs.com/package/@paraspell/assets

and used like this to get all assets in Acala for example

[andrew-ifrita]

while the asset-transfer-api might be deprecated, the asset-transfer-api-registry should still be kept alive and well maintained

afaik there is no alternative registry available in the ecosystem with same/equivalent level of information

No worries, as this is just a draft that is stage if we do deprecate. As mentioned by @franciscoaguirre and in the README changes there does exist an alternative. I'll take a deeper look to see how they compare and who will be impacted.

I see from this thread ParaLink is likely relying on this registry as well.

Luckily maintenance burden is pretty low here, so if this is more than an unused artifact of asset-transfer-api than it is probably fine to stick around. Although I do see some dependence on colorfulnotion/xcm-global-registry which has since disappeared. I can go through and make sure that hasn't had much of an impact and isn't causing a drift from reality.