Added notification banner to docs

pkg/yqlib/doc/operators/reduce.md

@@ -21,6 +21,12 @@ Reduce syntax in `yq` is a little different from `jq` - as `yq` (currently) isn'
 
 To that end, the reduce operator is called `ireduce` for backwards compatability if a `jq` like prefix version of `reduce` is ever added.
 
+{% hint style="warning" %}
+Note that versions prior to 4.18 require the 'eval/e' command to be specified. 
+
+`yq e <exp> <file>`
+{% endhint %}
+
 ## Sum numbers
 Given a sample.yml file of:
 ```yaml