pgen instructions in readme #15

Parser/grammar1.c: In function ‘PyGrammar_LabelRepr’:
Parser/grammar1.c:50:20: error: ‘_PyParser_TokenNames’ undeclared (first use in this function)
             return _PyParser_TokenNames[lb->lb_type];
                    ^
Parser/grammar1.c:50:20: note: each undeclared identifier is reported only once for each function it appears in
Parser/grammar1.c:61:1: warning: control reaches end of non-void function [-Wreturn-type]
 }