Add docs for addFieldFilter() function

Signed-off-by: David Arendsen <darendsen@gamepoint.com>

docs/04-functions.md

@@ -9,6 +9,7 @@ or [fromBucket()](functions/fromBucket.md)
 * [fromMeasurement()](functions/fromMeasurement.md)
 * [addAggregateWindow()](functions/addAggregateWindow.md)
 * [addDuplicate()](functions/addDuplicate.md)
+* [addFieldFilter()](functions/addFieldFilter.md)
 * [addGroup()](functions/addGroup.md)
 * [addLimit()](functions/addLimit.md)
 * [addMean()](functions/addMean.md)

docs/functions/addFieldFilter.md

@@ -0,0 +1,43 @@
+# Flux Query Builder Docs
+
+## Functions » addFieldFilter()
+
+### Parameters:
+
+<table>
+  <tbody>
+    <tr>
+      <th>Name</th>
+      <th>Required</th>
+      <th>Data type</th>
+      <th>Description</th>
+    </tr>
+    <tr>
+      <td>fields</td>
+      <td>Yes</td>
+      <td>array</td>
+      <td>
+        The fields you want to have a filter on.
+      </td>
+    </tr>
+  </tbody>
+</table>
+
+
+### Example
+
+```php
+->addFieldFilter(['field1', 'field2'])
+```
+
+This will result in the following Flux function part:
+
+```
+|> filter(fn: (r) => 
+    r._field == "field1" or r._field == "field2"
+)
+```
+
+### Extra resources
+
+* [Flux documentation](https://docs.influxdata.com/flux/v0.x/stdlib/universe/filter/)