nitcc: update README wrt Rejected

Signed-off-by: Jean Privat <jean@pryen.org>
nitcc: error if a token is rejected twice

Signed-off-by: Jean Privat <jean@pryen.org>
nitcc: error on duplicated expr/prod name

Signed-off-by: Jean Privat <jean@pryen.org>
nitcc: error on duplicated element name

Signed-off-by: Jean Privat <jean@pryen.org>
nitcc: error on duplicated alternative name

Signed-off-by: Jean Privat <jean@pryen.org>
nitcc: alttrans need no id

will add better transformations later (maybe)

Signed-off-by: Jean Privat <jean@pryen.org>