docs: Add docs for lang.Merge

See https://github.com/gohugoio/hugo/issues/4463

docs/content/functions/lang.Merge.md

@@ -0,0 +1,47 @@
+---
+title: lang.Merge
+description: "Merge missing translations from other languages."
+godocref: ""
+workson: []
+date: 2018-03-16
+categories: [functions]
+keywords: [multilingual]
+menu:
+  docs:
+    parent: "functions"
+toc: false
+signature: ["lang.Merge FROM TO"]
+workson: []
+hugoversion:
+relatedfuncs: []
+deprecated: false
+draft: false
+aliases: []
+comments:
+---
+
+As an example:
+
+```bash
+{{ $pages := .Site.RegularPages | lang.Merge $frSite.RegularPages | lang.Merge $enSite.RegularPages }}
+```
+
+Will "fill in the gaps" in the current site with, from left to right, content from the French site, and lastly the English.
+
+
+A more practical example is to fill in the missing translations for the "minority languages" with content from the main language:
+
+
+```bash
+ {{ $pages := .Site.RegularPages }}
+ {{ .Scratch.Set "pages" $pages }}
+ {{ $mainSite := .Sites.First }}
+ {{ if ne $mainSite .Site }}
+    {{ .Scratch.Set "pages" ($pages | lang.Merge $mainSite.RegularPages) }}
+ {{ end }}
+ {{ $pages := .Scratch.Get "pages" }} 
+ ```
+
+{{% note %}}
+Note that the slightly ugly `.Scratch` construct will not be needed once this is fixed: https://github.com/golang/go/issues/10608
+{{% /note %}}