README: Document our license-list version, parens issue, and license

The "at your option" language is modeled on Rust's from
rust-lang/rust@00c856c0 (Update license, add license boilerplate to
most files, 2012-12-03), although I've made it more compact by using
Markdown for the upstream license links.

I'd rather leave caller bugs to the issue tracker, but the paren bug
is an important issue for folks writing license expressions (i.e. all
crate authors), so I'm calling it out on our README until we get it
fixed.

There's an open SPDX issue about providing browsable archives for past
License List releases [1].  Until that's resolved, I'm just linking to
a tag in the release repository.

[1]: spdx/LicenseListPublisher#11

Cargo.toml

@@ -3,7 +3,7 @@ name = "license-exprs"
 version = "1.3.0"
 authors = ["Without Boats <woboats@gmail.com>"]
 license = "Apache-2.0 OR MIT"
-description = "Validate SPDX 2.1 license expressions."
+description = "Validate SPDX 2.1 license expressions using SPDX License List 3.0 identifiers."
 repository = "https://github.com/withoutboats/license-exprs"
 
 [[example]]

README.md

@@ -1 +1,17 @@
-This crate validates license strings as [SPDX 2.1 license expressions](https://spdx.org/spdx-specification-21-web-version#h.jxpfx0ykyb60).
+# license-exprs
+
+This crate validates [SPDX 2.1 license expressions][SPDX-license-expressions] using short identifiers from the [SPDX 3.0 License List][SPDX-license-list].
+
+## Limitations
+
+Parentheses are [not currently supported][parens].
+
+## License
+
+Licensed under the [Apache License, Version 2.0][Apache-2.0] ([`LICENSE-APACHE`](LICENSE-APACHE)) or the [MIT license][MIT] ([`LICENSE-MIT`](LICENSE-MIT)), at your option.
+
+[Apache-2.0]: https://www.apache.org/licenses/LICENSE-2.0
+[MIT]: https://opensource.org/licenses/MIT
+[parens]: https://github.com/rust-lang-nursery/license-exprs/issues/3
+[SPDX-license-expressions]: https://spdx.org/spdx-specification-21-web-version#h.jxpfx0ykyb60
+[SPDX-license-list]: https://github.com/spdx/license-list-data/tree/v3.0