Update transactions section

docs/architecture/adr-020-protobuf-client-encoding.md

@@ -23,6 +23,59 @@ querying.
 
 ## Decision
 
+### Transactions
+
+Since the messages that an application is known and allowed to handle are specific
+to the application itself, so must the transactions be specific to the application
+itself. Similar to how we described in [ADR 019](./adr-019-protobuf-state-encoding.md),
+the concrete types will be defined at the application level via Protobuf `oneof`.
+
+The application will define a single canonical `Message` Protobuf message
+with a single `oneof` that implements the SDK's `Msg` interface.
+
+Example:
+
+```protobuf
+// app/codec/codec.proto
+
+message Message {
+  option (cosmos_proto.interface_type) = "github.com/cosmos/cosmos-sdk/types.Msg";
+
+  oneof sum {
+    bank.MsgSend = 1;
+    staking.MsgCreateValidator = 2;
+    staking.MsgDelegate = 3;
+    // ...
+  }
+}
+```
+
+Because an application needs to define it's unique `Message` Protobuf message, it
+will by proxy have to define a `Transaction` Protobuf message that encapsulates this
+`Message` type. The `Transaction` message type must implement the SDK's `Tx` interface.
+
+Example:
+
+```protobuf
+// app/codec/codec.proto
+
+message Transaction {
+  option (cosmos_proto.interface_type) = "github.com/cosmos/cosmos-sdk/types.Tx";
+
+  StdTxBase base = 1;
+  repeated Message msgs = 2;
+}
+```
+
+Note, the `Transaction` type includes `StdTxBase` which will be defined by the SDK
+and includes all the core field members that are common across all transaction types.
+Developers do not have to include `StdTxBase` if they wish, so it is meant to be
+used as an auxiliary type.
+
+### Signing
+
+### CLI, REST, & Querying
+
 ## Consequences
 
 ### Positive