add select2-filter README and extension.json.

src/extensions/select2-filter/README.md

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+# Table Select2 Filter
+
+Use Plugin: [bootstrap-table-select2-filter](https://github.com/wenzhixin/bootstrap-table/tree/master/src/extensions/select2-filter) </br>
+Dependence if you use the select2 option: [Select2](https://select2.github.io/) v4.0.0
+
+## Usage
+
+```html
+<script src="extensions/select2-filter/bootstrap-table-select2-filter.js"></script>
+```
+
+## Options
+
+### filter
+
+* type: Boolean
+* description: enabled select2 filter exetension.
+* default: `false`
+
+### filterValues
+
+* type: Object
+* description: Set default selected value. <br>Example: {columnA.field.:'Column A Selected Value',columnB.field:'Column B Selected Value'}
+* default: `undefined`
+
+## Column options
+
+### filter
+
+* type: Object
+* description: Set filter option to configure the filter. <br>Example: {type:'select', data:["itemA", "itemB", "itemC"]}
+	* type: add an `input` or `select` into the column.
+	* data: need to set when type is `select` , filter data list. (The same as [Select2 Options](https://select2.github.io/))
+* default: `undefined`
+
+## Methods
+
+### setFilterData
+
+* Set column's filter data.
+	* <code> $table.bootstrapTable("setFilterData", "columnA.filed", ["itemA", "itemB", "itemC"]);</code>

src/extensions/select2-filter/extension.json

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+{
+  "name": "Select2 Filter",
+  "version": "1.0.0",
+  "description": "Plugin to add select2 filter on the top of the columns in order to filter the data.",
+  "url": "https://github.com/wenzhixin/bootstrap-table/tree/master/src/extensions/select2-filter",
+  "example": "http://issues.wenzhixin.net.cn/bootstrap-table/#extensions/select2-filter.html",
+
+  "plugins": [{
+    "name": "bootstrap-table-select2-filter",
+    "url": "https://github.com/wenzhixin/bootstrap-table/tree/master/src/extensions/select2-filter"
+  }],
+
+  "author": {
+    "name": "Jewway",
+    "image": "https://avatars0.githubusercontent.com/u/3501899"
+  }
+}