Add gitlens settings suggestion

While GitHub automatically uses this file if present, GitLens in VS Code
does not. Add the right option to our example settings.json for those
who use the extension.

Unfortunately, you can't leave this enabled if you want to look at the
repo _without_ the file; git blame just crashes when the file isn't
present. I'm not sure that there's a workaround for that.

.vscode/settings.template.json

@@ -6,4 +6,10 @@
 
     // To use the locally built compiler, after 'npm run build':
     // "typescript.tsdk": "built/local"
+
+    // To ignore commits listed in .git-blame-ignore-revs in GitLens:
+    // "gitlens.advanced.blame.customArguments": [
+    //     "--ignore-revs-file",
+    //     ".git-blame-ignore-revs"
+    // ]
 }