Document optional SField type magic

src/ripple/protocol/README.md

@@ -12,3 +12,29 @@ objects.
 
 The term "Tx" or "tx" is an abbreviation for "Transaction",
 a commonly occurring object type.
+
+### Optional Fields
+
+Our serialized fields have some "type magic" to make
+optional fields easier to read:
+
+- The operation `x[sfFoo]` means "return the value of 'Foo'
+  if it exists, or the default value if it doesn't."
+- The operation `x[~sfFoo]` means "return the value of 'Foo'
+  if it exists, or nothing if it doesn't." This usage of the
+  tilde/bitwise NOT operator is not standard outside of the
+  `rippled` codebase.
+    - As a consequence of this, `x[~sfFoo] = y[~sfFoo]`
+      assigns the value of Foo from y to x, including omitting
+      Foo from x if it doesn't exist in y.
+
+Typically, for things that are guaranteed to exist, you use
+`x[sfFoo]` and avoid having to deal with a container that may
+or may not hold a value. For things not guaranteed to exist,
+you use `x[~sfFoo]` because you want such a container. It
+avoids having to look something up twice, once just to see if
+it exists and a second time to get/set its value.
+([Real example](https://github.com/ripple/rippled/blob/35f4698aed5dce02f771b34cfbb690495cb5efcc/src/ripple/app/tx/impl/PayChan.cpp#L229-L236))
+
+The source of this "type magic" is in
+[SField.h](./SField.h#L296-L302).