更新README

增加简单算术类型验证码类
算术类型验证码可以设置难度、运算方法

src/test/java/com/wf/captcha/CaptchaTest.java

@@ -56,6 +56,8 @@ public class CaptchaTest {
         /*for (int i = 0; i < 10; i++) {
             ArithmeticCaptcha specCaptcha = new ArithmeticCaptcha();
             specCaptcha.setLen(3);
+            specCaptcha.supportAlgorithmSign(2);
+            specCaptcha.setDifficulty(50);
             specCaptcha.setFont(i, 28f);
             System.out.println(specCaptcha.getArithmeticString() + " " + specCaptcha.text());
             specCaptcha.out(new FileOutputStream(new File("C:/Java/aa" + i + ".png")));