Move docs to spec

spec/integrations/Bitcoin.md

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+# Bitcoin
+
+### Addresses
+
+Bitcoin addresses are an enum, defined as follows:
+
+  - `p2pkh`:  20-byte hash.
+  - `p2sh`:   20-byte hash.
+  - `p2wpkh`: 20-byte hash.
+  - `p2wsh`:  32-byte hash.
+  - `p2tr`:   32-byte key.
+
+### In Instructions
+
+Bitcoin In Instructions are present via the transaction's last output in the
+form of `OP_RETURN`, and accordingly limited to 80 bytes. `origin` is
+automatically set to the transaction's first input's address, if recognized.
+If it's not recognized, an address of the multisig's current Bitcoin address is
+used, causing any failure to become a donation.
+
+### Out Instructions
+
+Out Instructions ignore `data`.