Update README to include ISO code for Silesian

Source:

Hugging Face: https://huggingface.co/languages

README.md

@@ -23,9 +23,12 @@ task_categories:
 - translation
 language:
 - pl
+- szl
 pretty_name: Silesian-Polish MT benchmark
 size_categories:
 - n<1K
+tags:
+- machine-translation
 ---
 
-Polish-Silesian benchmark for evaluating MT models. Data was taken from bilingual websites and aligned by hand. 
+Polish-Silesian benchmark for evaluating MT models. Data was taken from bilingual websites and aligned by hand.