Adding documentation

site/docs/extensions/group-by.md

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+---
+layout: docs
+title: Table Group By
+description: Table Group By extension of Bootstrap Table.
+group: extensions
+toc: true
+---
+
+Use Plugin: [bootstrap-table-group-by3](https://github.com/wenzhixin/bootstrap-table/tree/master/src/extensions/boostrap-table-group-by) <br>
+You must include the bootstrap-table-group-by.css file in order to get the appropriate style
+
+## Usage
+
+{% highlight html %}
+<link rel="stylesheet" type="text/css" href="extensions/boostrap-table-group-by/bootstrap-table-group-by.css">
+<script src="extensions/boostrap-table-group-by/bootstrap-table-group-by.js"></script>
+{% highlight html %}
+
+jsFiddle example: [boostrap-table-group-by](https://jsfiddle.net/Bighamster/fj8d7kvn/)
+
+## Options
+
+### groupBy
+
+- **type:** `String`
+
+- **Detail:**
+
+   Set the name of the field that you want to group the data.
+
+- **Default:** ``
+
+### groupBySum
+
+- **type:** `String`
+
+- **Detail:**
+
+   Set the names of the fields by which you want to calculate sums.
+
+- **Default:** ``
+
+### groupByCount
+
+- **type:** `String`
+
+- **Detail:**
+
+   Set the names of the fields by which you want to calculate counts.
+
+- **Default:** ``
+
+### groupByAvg
+
+- **type:** `String`
+
+- **Detail:**
+
+   Set the names of the fields by which you want to calculate averages.
+
+- **Default:** ``