Added attributes' grammar
# Attributes
+> **<sup>Syntax</sup>**
+> _Attribute_ :
+> &nbsp;&nbsp; _InnerAttribute_ | _OuterAttribute_
+>
+> _InnerAttribute_ :
+> &nbsp;&nbsp; `#![` MetaItem `]`
+>
+> _OuterAttribute_ :
+> &nbsp;&nbsp; `#[` MetaItem `]`
+>
+> _MetaItem_ :
+> &nbsp;&nbsp; &nbsp;&nbsp; IDENTIFIER
+> &nbsp;&nbsp; | IDENTIFIER `=` LITERAL
+> &nbsp;&nbsp; | IDENTIFIER `(` _MetaSeq_ `)`
+> &nbsp;&nbsp; | IDENTIFIER `(` _MetaSeq_ `,` `)`
+>
+> _MetaSeq_ :
+> &nbsp;&nbsp; &nbsp;&nbsp; EMPTY
+> &nbsp;&nbsp; | _MetaItem_
+> &nbsp;&nbsp; | _MetaSeq_ `,` _MetaItem_
+
Any item declaration may have an _attribute_ applied to it. Attributes in Rust
are modeled on Attributes in ECMA-335, with the syntax coming from ECMA-334
(C#). An attribute is a general, free-form metadatum that is interpreted
