add object descriptions for accessibility

src/main/res/layout/participate_layout.xml

@@ -101,7 +101,8 @@
                         android:background="@color/white"
                         android:onClick="onGetRCPlayStoreClick"
                         android:padding="@dimen/zero"
-                        android:src="@drawable/playstore"/>
+                        android:src="@drawable/playstore"
+                        android:contentDescription="@string/participate_rc_play_store"/>
 
                     <ImageButton
                         android:id="@+id/participate_release_candidate_fdroid"
@@ -110,7 +111,8 @@
                         android:background="@color/white"
                         android:onClick="onGetRCFDroidClick"
                         android:padding="@dimen/zero"
-                        android:src="@drawable/fdroid"/>
+                        android:src="@drawable/fdroid"
+                        android:contentDescription="@string/participate_rc_fdroid"/>
 
                 </LinearLayout>
 
@@ -139,7 +141,8 @@
                     android:background="@color/white"
                     android:onClick="onGetBetaFDroidClick"
                     android:padding="@dimen/zero"
-                    android:src="@drawable/fdroid"/>
+                    android:src="@drawable/fdroid"
+                    android:contentDescription="@string/participate_dev_fdroid"/>
 
                 <ImageButton
                     android:id="@+id/participate_beta_apk"
@@ -148,7 +151,8 @@
                     android:background="@color/white"
                     android:onClick="onGetBetaApkClick"
                     android:padding="@dimen/zero"
-                    android:src="@drawable/apk"/>
+                    android:src="@drawable/apk"
+                    android:contentDescription="@string/participate_dev_direct_download"/>
 
                 </LinearLayout>
 

src/main/res/values/strings.xml

@@ -588,6 +588,10 @@
     <string name="participate_contribute_translate_link" translatable="false">&lt;font color=\"%1$s\">&lt;a href=\"%2$s\">%3$s&lt;/a>&lt;/font></string>
     <string name="participate_contribute_github_text">Review, amend and write code, see %1$s for details</string>
     <string name="participate_contribute_github_text_link" translatable="false">&lt;font color=\"%1$s\">&lt;a href=\"%2$s\">CONTRIBUTING.md&lt;/a>&lt;/font></string>
+    <string name="participate_rc_play_store">Get release candidate via Google Play store</string>
+    <string name="participate_rc_fdroid">Get release candidate via f-droid app store</string>
+    <string name="participate_dev_fdroid">Get development release via f-droid app store</string>
+    <string name="participate_dev_direct_download">Get development release via direct download</string>
     <string name="move_to">Move to&#8230;</string>
     <string name="copy_to">Copy to&#8230;</string>
     <string name="choose_remote_folder">Choose remote folder&#8230;</string>