Update README.md

I've added an Assamese translation link (HOWTO-as.md) of HOWTO.md (english file).

docs/README.md

@@ -6,6 +6,8 @@ Volunteers have translated many of our Contributing, How-to, and Code of Conduct
 
 - Arabic / al arabiya / العربية
   - [How-to](HOWTO-ar.md)
+- Assamese / অসমীয়া
+  - [How-to](HOWTO-as.md)
 - Azerbaijani / Азәрбајҹан дили / آذربايجانجا ديلي
 - Bengali / বাংলা
   - [Code of Conduct](CODE_OF_CONDUCT-bn.md)