Added missing docs on trace

docs/reference/functions/trace.md

@@ -0,0 +1,45 @@
+# Function trace
+
+Calculate the trace of a matrix: the sum of the elements on the main
+diagonal of a square matrix.
+
+
+## Syntax
+
+```js
+math.trace(x)
+```
+
+### Parameters
+
+Parameter | Type | Description
+--------- | ---- | -----------
+`x` | Array | Matrix | A matrix
+
+### Returns
+
+Type | Description
+---- | -----------
+Number | The trace of `x`
+
+
+## Examples
+
+```js
+math.trace([[1, 2], [3, 4]]); // returns 5
+
+var A = [
+  [1, 2, 3],
+  [-1, 2, 3],
+  [2, 0, 3]
+]
+math.trace(A); // returns 6
+```
+
+
+## See also
+
+[diag](diag.md)
+
+
+<!-- Note: This file is automatically generated from source code comments. Changes made in this file will be overridden. -->