docs(combo): update feat/README and CHANGELOG

Author: ViktorSlavov

CHANGELOG.md

@@ -82,6 +82,14 @@ All notable changes for each version of this project will be documented in this
 - `IgxDateTimeEditor`, `IgxMask`, `IgxDatePicker`, `IgxTimePicker`, `IgxDateRangePicker`
     - Added IME input support. When typing in an Asian language input, the control will display input method compositions and candidate lists directly in the control’s editing area, and immediately re-flow surrounding text as the composition ends.
 
+- `IgxCombo`
+    - Added `selectionMode` `@Input`. Allows to specify whether the selection mode should be 'multiple' (default, current behavior) or 'single' (allowing only one item to be selected).
+    ```html
+    <igx-combo #single [data]="myData" selectionMode="single">
+        ...
+    </igx-combo>
+    ``` 
+
 ### General
 - `IgxGridComponent`
     - The following properties are deprecated:

projects/igniteui-angular/src/lib/combo/README.md

@@ -301,6 +301,7 @@ Setting `[displayDensity]` affects the control's items' and inputs' css properti
 |  `id`                    | combo id                                          | string                      |
 |  `data`                  | combo data source                                 | any                         |
 |  `allowCustomValue`      | enables/disables combo custom value                | boolean                     |
+|  `selectionMode`         | specified whether the combo allows single or multiple selection | string ('multiple' | 'single' ) | 
 |  `filterable`            | enables/disables combo drop down filtering - enabled by default                  | boolean                     |
 |  `showSearchCaseIcon`          | defines whether the search case-sensitive icon should be displayed - disabled by default| boolean                     |
 |  `valueKey`              | combo value data source property                  | string                      |