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 --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index d3ff879c33af5518a5521b6f239d1a295663977c..cb0618b15fdd528e6c7c9510f6ebfd50638febe6 100644 (file)
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -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