Move rust-selectors in-tree.

components/selectors/README.md

@@ -0,0 +1,25 @@
+rust-selectors
+==============
+
+* [![Build Status](https://travis-ci.org/servo/rust-selectors.svg?branch=master)](
+  https://travis-ci.org/servo/rust-selectors)
+* [Documentation](https://docs.rs/selectors/)
+* [crates.io](https://crates.io/crates/selectors)
+
+CSS Selectors library for Rust.
+Includes parsing and serilization of selectors,
+as well as matching against a generic tree of elements.
+Pseudo-elements and most pseudo-classes are generic as well.
+
+**Warning:** breaking changes are made to this library fairly frequently
+(13 times in 2016, for example).
+However you can use this crate without updating it that often,
+old versions stay available on crates.io and Cargo will only automatically update
+to versions that are numbered as compatible.
+
+To see how to use this library with your own tree representation,
+see [Kuchiki’s `src/select.rs`](https://github.com/SimonSapin/kuchiki/blob/master/src/select.rs).
+(Note however that Kuchiki is not always up to date with the latest rust-selectors version,
+so that code may need to be tweaked.)
+If you don’t already have a tree data structure,
+consider using [Kuchiki](https://github.com/SimonSapin/kuchiki) itself.