Auto merge of rust-lang#2767 - oli-obk:josh_auth, r=RalfJung

Explain how to make josh authenticate with github via ssh

Without this I was unable to push, even though pushing directly to https without josh works.

src/tools/miri/CONTRIBUTING.md

@@ -242,6 +242,13 @@ 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.
 
+To make josh push via ssh instead of https, you can add 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.