Add documentation for cond function.

Fixes #180.

content/functions/cond.md

@@ -0,0 +1,26 @@
+---
+title: "cond"
+date: 2017-09-08
+description: "Return one of two arguments, depending on the value of a third argument."
+categories: [functions]
+menu:
+  docs:
+    parent: "functions"
+signature: ["cond CONTROL VAR1 VAR2"]
+aliases: [/functions/cond/]
+hugoversion: 0.27
+relatedfuncs: [default]
+toc: false
+draft: false
+needsexamples: false
+---
+
+`cond` returns *VAR1* if *CONTROL* is true, or *VAR2* if it is not.
+
+Example:
+
+```
+{{ cond (eq (len $geese) 1) "goose" "geese" }}
+```
+
+Would emit "goose" if the `$geese` array has exactly 1 item, or "geese" otherwise.