Add sum() Flux function + docs

Signed-off-by: davidarendsen <davidarendsen@hey.com>

docs/functions/addSum.md

@@ -0,0 +1,39 @@
+# Flux Query Builder Docs
+
+## Functions » addSum()
+
+### Parameters:
+
+<table>
+  <tbody>
+    <tr>
+      <th>Name</th>
+      <th>Required</th>
+      <th>Data type</th>
+      <th>Description</th>
+    </tr>
+    <tr>
+      <td>column</td>
+      <td>Yes</td>
+      <td>string</td>
+      <td>Column to operate on. Default is "_value".</td>
+    </tr>
+  </tbody>
+</table>
+
+
+### Example
+
+```php
+->addSum('_value')
+```
+
+This will result in the following Flux function part:
+
+```
+|> sum(column: "_value")
+```
+
+### Extra resources
+
+* [Flux documentation](https://docs.influxdata.com/flux/v0.x/stdlib/universe/sum/)