Update app.py with demo application description

app.py

@@ -60,6 +60,13 @@ def translate(text: str, src_lang: str, tgt_lang: str):
 
 description = """
 No Language Left Behind (NLLB) is a series of open-source models aiming to provide high-quality translations between 200 language.
+This demo application allows you to use the NLLB model to translate text between a source and target language.
+
+## Notes 
+
+- Whilst the model supports 200 languages, the quality of translations may vary between languages. 
+- "Low Resource" languages (languages which are less present on the internet and have a lower amount of investment) may have lower quality translations.
+- The demo is not intended to be used for very long texts. 
 """