github: ensure using the right branch of Home Manager

.github/ISSUE_TEMPLATE/issue.yaml

@@ -8,6 +8,17 @@ labels: [bug, triage]
 assignees: [rycee, berbiche, sumnerevans]
 
 body:
+- type: checkboxes
+  attributes:
+    label: Are you following the right branch?
+    description: |
+      You should follow the branch of Home Manager that corresponds to your
+      version of Nixpkgs; see
+      [the README](https://github.com/nix-community/home-manager#releases)
+      for details.
+    options:
+    - label: My Nixpkgs and Home Manager versions are in sync
+      required: true
 - type: checkboxes
   attributes:
     label: Is there an existing issue for this?