Add section for foreign assets to the assets integrations guide (#5832)

* Add section for foreign assets

* Update docs/build/build-integrate-assets.md

---------

Co-authored-by: Radha <86818441+DrW3RK@users.noreply.github.com>

docs/build/build-integrate-assets.md

@@ -75,6 +75,14 @@ Note that you can use the same addresses (except
 you use on the Relay Chain. The SS58 encodings are the same; only the chain information (genesis
 hash, etc.) will change on transaction construction.
 
+### Foreign Assets
+
+Foreign assets are those assets in Asset Hub whose native blockchain is not Asset Hub. These are mainly native tokens from other parachains or bridged tokens from other consensus systems (such as Ethereum). Once a foreign asset has been registered in Asset Hub (by its root origin), users are enabled to send this token from its native blockchain to Asset Hub and operate with it as if it were any other asset.
+
+Practically speaking, foreign assets are handled by the `foreign-assets` pallet in Asset Hub, which is an instance of the Assets pallet. Hence, this pallet exposes the same interface to users and other pallets as the Assets pallet.
+
+The main difference to take into account for foreign assets is their identifier. Instead of using integers as identifiers like in the Assets pallet, assets stored in the `foreign-assets` pallet are identified by [their XCM multilocation](https://wiki.polkadot.network/docs/learn/xcm/fundamentals/multilocation-summary).
+
 ## Integration
 
 The Asset Hub will come with the same tooling suite that Parity Technologies provides for the Relay