Add missing localization keys for custom entry types

Author: Memeel

jablib/src/main/resources/l10n/JabRef_en.properties

@@ -2270,6 +2270,10 @@ Required=Required
 Entry\ type\ cannot\ be\ empty\ and\ must\ not\ contain\ spaces.=Entry type cannot be empty and must not contain spaces.
 Field\ cannot\ be\ empty.\ Please\ enter\ a\ name.=Field cannot be empty. Please enter a name.
 
+Add\ new\ custom\ field=Add new custom field
+Field\ cannot\ be\ empty\ and\ must\ not\ contain\ spaces.=Field cannot be empty and must not contain spaces.
+Type\ new\ custom\ field...=Type new custom field...
+
 Capitalize\ current\ word=Capitalize current word
 Delete\ text=Delete text
 Make\ current\ word\ lowercase=Make current word lowercase