addd placeholder in conversation search input

_locales/en/messages.json

@@ -2390,5 +2390,8 @@
   },
   "compose": {
     "message": "Compose"
+  },
+  "searchForAKeyPhrase": {
+    "message": "Search for a key phrase or contact"
   }
 }

ts/components/session/SessionConversationSearch.tsx

@@ -23,6 +23,7 @@ export class SessionConversationSearch extends React.Component<Props> {
         <input
           value={searchString}
           onChange={e => this.props.onChange(e.target.value)}
+          placeholder={window.i18n('searchForAKeyPhrase')}
         />
       </div>
     );