xso: add README files

xso-proc/README.md

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+xso_proc -- derive macros for XML parsing
+=========================================
+
+What’s this?
+------------
+
+This crate provides the derive macros exported by the
+[xso](https://crates.io/crates/xso) crate. Please see that crate's README and
+documentation for more information.
+
+What license is it under?
+-------------------------
+
+MPL-2.0 or later, see the `LICENSE` file.

xso/README.md

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+xso -- serde-like parsing for XML
+=================================
+
+What’s this?
+------------
+
+This crate provides the traits for parsing XML data into Rust structs, and
+vice versa. You can do things like:
+
+```rust
+#[derive(FromXml, AsXml)]
+#[xml(namespace = "urn:example", name = "element")]
+pub struct Foo;
+```
+
+For more information, see
+[its documentation on docs.rs](https://docs.rs/xso/latest/xso/) for the latest
+release or
+[the documentation for the main branch on our servers](https://docs.xmpp.rs/main/xso/).
+
+What license is it under?
+-------------------------
+
+MPL-2.0 or later, see the `LICENSE` file.