message: Update CONTRIBUTING.md
  Point out that code added to Cargo may raise the minimal version of Rust required. Cargo does not have a grace period where it has to compile on "stable -2" or anything like that.
diff: @@ -18,6 +18,10 @@ installed locally, ignore the cross-compile test failures, or disable them by
  using `CFG_DISABLE_CROSS_TESTS=1 cargo test`. Note that some tests are enabled
  only on nightly toolchain.
  +## Minimal version of Rust
  +
  +Cargo must compile on stable Rust. Code added to Cargo is encouraged to use
  +the latest stable features of the language and `stdlib`.
  ## Contributing to the Docs
  