Add boolean literal to the grammar

To make this rule match the definition of Lit_ in
https://github.com/rust-lang/rust/blob/master/src/libsyntax/ast.rs

Author: carols10cents

src/doc/grammar.md

@@ -176,7 +176,7 @@ excluded from the `ident` rule.
 
 ```antlr
 lit_suffix : ident;
-literal : [ string_lit | char_lit | byte_string_lit | byte_lit | num_lit ] lit_suffix ?;
+literal : [ string_lit | char_lit | byte_string_lit | byte_lit | num_lit | bool_lit ] lit_suffix ?;
 ```
 
 #### Character and string literals
@@ -238,7 +238,9 @@ dec_lit : [ dec_digit | '_' ] + ;
 
 #### Boolean literals
 
-**FIXME:** write grammar
+```antlr
+bool_lit : [ "true" | "false" ] ;
+```
 
 The two values of the boolean type are written `true` and `false`.