xso-proc: start making derive macros for FromXml and IntoXml

For now, these macros only support empty elements. Everything else will
be rejected with a compile-time error.

xso/src/from_xml_doc.md

@@ -0,0 +1,50 @@
+# Make a struct or enum parseable from XML
+
+This derives the [`FromXml`] trait on a struct or enum. It is the counterpart
+to [`macro@IntoXml`].
+
+## Example
+
+```rust
+# use xso::FromXml;
+static MY_NAMESPACE: &str = "urn:example";
+
+#[derive(FromXml, Debug, PartialEq)]
+#[xml(namespace = MY_NAMESPACE, name = "foo")]
+struct Foo;
+
+let foo: Foo = xso::from_bytes(b"<foo xmlns='urn:example'/>").unwrap();
+assert_eq!(foo, Foo);
+```
+
+## Attributes
+
+The derive macros need to know which XML namespace and name the elements it
+is supposed have. This must be specified via key-value pairs on the type the
+derive macro is invoked on. These are specified as Rust attributes. In order
+to disambiguate between XML attributes and Rust attributes, we are going to
+refer to Rust attributes using the term *meta* instead, which is consistent
+with the Rust language reference calling that syntax construct *meta*.
+
+All key-value pairs interpreted by these derive macros must be wrapped in a
+`#[xml( ... )]` *meta*. The following keys are defined on structs:
+
+| Key | Value type | Description |
+| --- | --- | --- |
+| `namespace` | *path* | The path to a `&'static str` which holds the XML namespace to match. |
+| `name` | *string literal* | The XML element name to match. |
+
+## Limitations
+
+Supports only empty structs currently. For example, the following will not
+work:
+
+```compile_fail
+# use xso::FromXml;
+# static MY_NAMESPACE: &str = "urn:example";
+#[derive(FromXml, Debug, PartialEq)]
+#[xml(namespace = MY_NAMESPACE, name = "foo")]
+struct Foo {
+    some_field: String,
+}
+```