Move docs to spec

spec/integrations/Ethereum.md

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+# Ethereum
+
+### Addresses
+
+Ethereum addresses are 20-byte hashes.
+
+### In Instructions
+
+Ethereum In Instructions are present via being appended to the calldata
+transferring funds to Serai. `origin` is automatically set to the party from
+which funds are being transferred. For an ERC20, this is `from`. For ETH, this
+is the caller.
+
+### Out Instructions
+
+`data` is limited to 512 bytes.
+
+If `data` is provided, the Ethereum Router will call a contract-calling child
+contract in order to sandbox it. The first byte of `data` designates which child
+child contract to call. After this byte is read, `data` is solely considered as
+`data`, post its first byte. The child contract is sent the funds before this
+call is performed.
+
+##### Child Contract 0
+
+This contract is intended to enable connecting with other protocols, and should
+be used to convert withdrawn assets to other assets on Ethereum.
+
+  1) Transfers the asset to `destination`.
+  2) Calls `destination` with `data`.
+
+##### Child Contract 1
+
+This contract is intended to enable authenticated calls from Serai.
+
+  1) Transfers the asset to `destination`.
+  2) Calls `destination` with `data[.. 4], serai_address, data[4 ..]`, where
+`serai_address` is the address which triggered this Out Instruction.