delete recruitment information

Minimum supported idea version up to 2018.3
Kotlin upgrade to 1.3.72

嵩山版于2020年8月3日正式发布

更新为嵩山版

Update：华山 --> 泰山

更新gitbook说明

已有最新泰山版，删除老的华山版

1.6.0版本，发布于2020.4.22

remove unless content

2. fix some issue

同步改动

fix https://github.com/alibaba/p3c/issues/209

更新开发手册至华山版

1. pmd -> 6.15.0
2. idea min version 2016.1
3. jdk min version 1.8

1. pmd -> 6.15.0
2. idea min version 2016.1
3. jdk min version 1.8

fix typo

update new rules in readme

This fixes https://github.com/alibaba/p3c/issues/471

Previously, the following code can't pass EqualsAvoidNullRule:

public class Test {
  private static final String VERSION = System.getProperty("v");
  public boolean isJava6(){
    return "1.6".equals(VERSION);
  }
}

This PR fixes this issue by checking if the caller is a literal.

Update README.md