Add a README to libtest with a roadmap

Author: gnzlbg

src/libtest/README.md

@@ -0,0 +1,30 @@
+WIP - stable libtest
+===
+
+The migration of libtest to stable Rust is currently in progress.
+
+You can find libtest at: https://github.com/rust-lang/libtest . If you need to
+make a change:
+
+* perform the change there, 
+* do a new crates.io release, and
+* send a PR to rust-lang/rust bumping the libtest version.
+
+## Roadmap
+
+Right now all the contests of libtest live in the external repo. 
+
+The next steps are:
+
+* make `#[test]` and `#[ignore]` procedural macros in the prelude by default,
+  routed to the same procedural macro, so that it doesn't matter which one runs
+  first.
+* move the unstable APIs back into rust-lang/rust to help maintainability
+  (replacing `pub use libtest::*` with explicit imports)
+* migrate libtest to the real `term` crate
+* provide `libtest` a real `custom_test_framework` runner (in parallel with the
+  runner in rust-lang/rust)
+* set up `libtest` as a normal `custom_test_framework` inside rust-lang/rust
+* refactor the internal structure of `libtest` to make it re-usable by
+  third-party custom test frameworks (think test formatting, benchmark
+  formatting, argument parsing, json format serialization, etc.)