docs: add java static constants conventions to README.md

Author: tom-weelborg

README.md

@@ -83,4 +83,6 @@ The following are lists of conventions that either *should* be followed or *must
     - Instance attributes
     - Constructors
     - `static` functions
-    - Instance methods
+    - Instance methods
+- `static` Constants:
+  - Names must be `SCREAMING_SNAKE_CASE`