Add button to help to translate peertube

client/src/app/menu/language-chooser.component.html

@@ -4,6 +4,11 @@
     <span class="close" aria-label="Close" role="button" (click)="hide()"></span>
   </div>
 
+
+  <a i18n class="help-to-translate" target="_blank" rel="noreferrer noopener" href="https://github.com/Chocobozzz/PeerTube/blob/develop/support/doc/translation.md">
+    Help to translate PeerTube!
+  </a>
+
   <div class="modal-body">
     <a *ngFor="let lang of languages" [href]="buildLanguageLink(lang)">{{ lang.label }}</a>
   </div>

client/src/app/menu/language-chooser.component.scss

@@ -1,6 +1,11 @@
 @import '_variables';
 @import '_mixins';
 
+.help-to-translate {
+  @include peertube-button-link;
+  @include orange-button;
+}
+
 .modal-body {
   text-align: center;