Add a note for using ssh login with josh

Author: oli-obk

src/tools/miri/CONTRIBUTING.md

@@ -242,6 +242,14 @@ josh-proxy --local=$HOME/.cache/josh --remote=https://github.com --no-background
 
 This uses a directory `$HOME/.cache/josh` as a cache, to speed up repeated pulling/pushing.
 
+Note that josh is unable to handle ssh auth natively, but you can force it to use ssh auth
+by adding the following to your `.gitconfig`:
+
+```toml
+[url "git@github.com:"]
+    pushInsteadOf = https://github.com/
+```
+
 ### Importing changes from the rustc repo
 
 Josh needs to be running, as described above.