Added user defined attribute declarations.

This is such that if you mis-spell an attribute, it doesn't get
silently ignored.

Bug: 18294628
Change-Id: I10013f5b2a21048b7daba2e9410678f528e09761
Tested: on Linux.

docs/source/Schemas.md

@@ -9,6 +9,8 @@ first:
 
     namespace MyGame;
 
+    attribute "priority";
+
     enum Color : byte { Red = 1, Green, Blue }
 
     union Any { Monster, Weapon, Pickup }
@@ -211,7 +213,8 @@ in the corresponding C++ code. Multiple such lines per item are allowed.
 Attributes may be attached to a declaration, behind a field, or after
 the name of a table/struct/enum/union. These may either have a value or
 not. Some attributes like `deprecated` are understood by the compiler,
-others are simply ignored (like `priority` in the example above), but are
+user defined ones need to be declared with the attribute declaration
+(like `priority` in the example above), and are
 available to query if you parse the schema at runtime.
 This is useful if you write your own code generators/editors etc., and
 you wish to add additional information specific to your tool (such as a