Added class description for MaterialsDescription

Author: Anirav Kareddy

src/main/java/software/amazon/encryption/s3/materials/MaterialsDescription.java

@@ -6,6 +6,11 @@
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.Map;
+/**
+ * This class is used to store and manage key-value pairs that describe keyring,specifically for AES and RSA Keyring.
+ * This will be useful during re-encryption of instruction file.
+ * The stored Materials Description are immutable once created.
+ */
 
 public class MaterialsDescription {
     private final Map<String, String> materialsDescription;