[librsvg/librsvg-2.44] CONTRIBUTING.md: Update rustfmt documentation

[Federico Mena Quintero <federico src gnome org>]

commit 0ce95d72e4807c5bbb5504e20b6a926c5a3c4bc2
Author: Jordan Petridis <jordanpetridis protonmail com>
Date:   Fri Sep 14 22:45:39 2018 +0000

    CONTRIBUTING.md: Update rustfmt documentation

 CONTRIBUTING.md | 13 ++++---------
 1 file changed, 4 insertions(+), 9 deletions(-)
---
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 1df6ec92..d54a99a2 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -161,19 +161,14 @@ of CI pipelines will fail:
     indentation, but don't want it to make changes on its own:  
     `cargo fmt --all -- --check`
 
-***Installing rustfmt*** As of 2018/Jun, our continuous integration
-pipeline assumes the Rust nightly version of rustfmt.  You can install
-it with
+***Installing rustfmt*** As of 2018/Sep, our continuous integration
+pipeline assumes the version of rustfmt that is distributed through the
+nightly channel of [rustup](rustup.rs).  You can install it with
 
 ```
-cargo +nightly install --force rustfmt-nightly
+rustup component add rustfmt-preview --toolchain=nightly
 ```
 
-Note that rustfmt changes frequently.  If the CI pipeline fails on the
-`Lint` stage because your code is formatted differently, try updating
-your rustfmt.  Hopefully this will stabilize once rustfmt reaches
-version 1.0.
-
 ### Test suite
 
 Please make sure that the test suite passes with the changes in your