docs: Document symdiff

Closes gohugoio/hugoDocs#659

docs/content/en/functions/symdiff.md

@@ -0,0 +1,28 @@
+---
+title: "symdiff"
+description: "`collections.SymDiff` (alias `symdiff`) returns the symmetric difference of two collections."
+date: 2018-11-07
+categories: [functions]
+menu:
+  docs:
+    parent: "functions"
+keywords: [collections,intersect,union, complement]
+signature: ["COLLECTION | symdiff COLLECTION" ]
+hugoversion: "0.51"
+aliases: []
+---
+
+Example:
+
+```go-html-template
+{{ slice 1 2 3 | symdiff (slice 3 4) }}
+```
+
+The above will print `[1 2 4]`.
+
+Also see https://en.wikipedia.org/wiki/Symmetric_difference
+
+
+
+
+