Describe how to patch Nix expressions when CI fails (#717)

- Add instructions to README describing under what circumstances the
  patch becomes available, what it does, and how to apply it.

Author: sevanspowell

README.md

@@ -65,6 +65,18 @@ nix-shell
 sbtix-gen-all2
 ```
 
+OR
+
+If the "ensure Nix expressions are up-to-date" step of your CI
+build has failed, check the artifacts of that step. There should be a
+patch provided, which you can apply locally with:
+
+```
+patch -p1 < downloaded.patch
+```
+
+This patch will update the lock files for you.
+
 ###### Why so many lock files?
 
 - `repo.nix`                 : generated by the `sbtix-gen` command and includes only the build dependencies for the project.