Support for missing strings fallback

[guillaumeboehm]

Opening a new pr following #46 with the feature as an opt-in option, updated examples, and for the wiki the only relevant change seems to be this:

diff --git a/1.-Installation,-Configuration-&-Usage.md b/1.-Installation,-Configuration-&-Usage.md
index 40c1400..34608d0 100644
--- a/1.-Installation,-Configuration-&-Usage.md
+++ b/1.-Installation,-Configuration-&-Usage.md
@@ -55,6 +55,15 @@ If the assets directory for the localization files is different than the default
       ...
 ```
 
+If you want your translations to use the fallback locale in case of missing strings in the translation files you can set this option (default is ```false```):
+
+```dart
+ var delegate = await LocalizationDelegate.create(
+      ...
+        useFallbackForMissingStrings: true
+      ...
+```
+
 Example MyApp:
 
 ```dart