Clarify initializer documentation

Make it clear that there is only one initializer in the whole grammar.
The previous formulation could have been understood to mean that there
can be an initializer for every rule in the grammar.

Fixes #82.

README.md

@@ -182,14 +182,14 @@ written as a JavaScript string between the name and separating equality sign.
 Rules need to be separated only by whitespace (their beginning is easily
 recognizable), but a semicolon (“;”) after the parsing expression is allowed.
 
-Rules can be preceded by an *initializer* — a piece of JavaScript code in curly
+The first rule can be preceded by an *initializer* — a piece of JavaScript code
-braces (“{” and “}”). This code is executed before the generated parser starts
+in curly braces (“{” and “}”). This code is executed before the generated parser
-parsing. All variables and functions defined in the initializer are accessible
+starts parsing. All variables and functions defined in the initializer are
-in rule actions and semantic predicates. The code inside the initializer can
+accessible in rule actions and semantic predicates. The code inside the
-access the parser object using the `parser` variable and options passed to the
+initializer can access the parser object using the `parser` variable and options
-parser using the `options` variable. Curly braces in the initializer code must
+passed to the parser using the `options` variable. Curly braces in the
-be balanced. Let's look at the example grammar from above using a simple
+initializer code must be balanced. Let's look at the example grammar from above
-initializer.
+using a simple initializer.
 
     {
       function makeInteger(o) {