Extend instructions with download command for git-subtree script

Author: oli-obk

CONTRIBUTING.md

@@ -292,5 +292,7 @@ Every time:
 
 ```
 ulimit -s 60000
-src/tools/miri/git-subtree.sh push --prefix src/tools/miri/ miri-your-fork miri-subtree-sync
+wget https://raw.githubusercontent.com/gitgitgadget/git/fe2e4819b869725f870cd3ce99f1f8150fe17dc1/contrib/subtree/git-subtree.sh
+sh git-subtree.sh push --prefix src/tools/miri/ miri-your-fork miri-subtree-sync
+rm git-subtree.sh
 ```