Add some basic info about the serialization fields

packages/ripple-binary-codec/src/enums/README.md

@@ -0,0 +1,44 @@
+# Definitions
+
+## Types
+
+TODO
+
+## Ledger Entry Types
+
+TODO
+
+## Fields
+
+These are Serialization Fields (`sf`) [defined in rippled's SField.cpp](https://github.com/ripple/rippled/blob/develop/src/ripple/protocol/impl/SField.cpp).
+
+### Key
+
+The key is the string defined in the rippled source code, such as "LedgerEntry", "Transaction", etc.
+
+### nth
+
+nth is the `index` used to make an [`SField` field code](https://github.com/ripple/rippled/blob/eaff9a0e6aec0ad077f118501791c7684debcfd5/src/ripple/protocol/SField.h#L95-L98).
+
+### isVLEncoded
+
+If true, the field is Variable Length encoded. The variable-length encoded fields are `STI_VL`/`Blob`, `STI_ACCOUNT`/`AccountID`, and `STI_VECTOR256`/`Vector256`.
+
+### isSerialized
+
+Fields are serialized if they are not [one of these](https://github.com/ripple/rippled/blob/eaff9a0e6aec0ad077f118501791c7684debcfd5/src/ripple/protocol/impl/SField.cpp#L71-L78) or if they are not an SField.
+
+- https://github.com/ripple/ripple-binary-codec/blob/master/src/enums/definitions.json#L832
+- https://github.com/ripple/rippled/search?utf8=%E2%9C%93&q=taker_gets_funded&type=
+
+### isSigningField
+
+True unless the field is [specified with `SField::notSigning`](https://github.com/ripple/rippled/blob/eaff9a0e6aec0ad077f118501791c7684debcfd5/src/ripple/protocol/impl/SField.cpp#L198).
+
+## Transaction Results
+
+TODO
+
+## Transaction Types
+
+TODO