Update README.md to reflect changes to features

Author: Munksgaard

README.md

@@ -17,12 +17,18 @@ To use `compiletest-rs` in your application, add the following to `Cargo.toml`
 compiletest_rs = "*"
 ```
 
-This will require you to use rust nightly.  If you want to use rust stable
-set the feature `stable`:
+By default, `compiletest-rs` should be able to run on both stable, beta and
+nightly channels of rust. We use the [`tester`](tester) fork of Rust's builtin
+`test` crate, so that we don't have require nightly. If you are running nightly
+and want to use Rust's `test` crate directly, you to have the rustc development
+libraries install (which you can get by running `rustup component add rustc-dev
+--target nightly`). Once you have the rustc development libraries installed, you
+can use the `rustc` feature to make compiletest use them instead of the `tester`
+crate.
 
 ```toml
 [dev-dependencies]
-compiletest_rs = { version = "*", features = [ "stable" ] }
+compiletest_rs = { version = "*", features = [ "rustc" ] }
 ```
 
 Create a `tests` folder in the root folder of your project. Create a test file
@@ -112,3 +118,4 @@ If you are unsure, open a pull request anyway and we would be glad to help!
 [upstream]: https://github.com/rust-lang/rust/tree/master/src/tools/compiletest
 [src]: https://github.com/rust-lang/rust/tree/master/src/tools/compiletest/src
 [tests]: https://rust-lang.github.io/rustc-guide/tests/adding.html#header-commands-configuring-rustc
+[tester]: https://crates.io/crates/tester