docs: update readme guideline

Author: yanglbme

README.md

@@ -115,12 +115,13 @@
 如果你对本项目感兴趣，并且希望加入我们刷题小分队，欢迎随时提交 [PR](https://github.com/doocs/leetcode/pulls)。请参考如下步骤：
 
 1. 将本项目 fork 到你的个人 GitHub 帐户，然后 clone 到你的本地机器；
+1. 进入 leetcode 目录，切换到一个新的分支；
 1. 对项目做出一些变更，然后使用 git add、commit、push 等命令将你的本地变更提交到你的远程 GitHub 仓库；
 1. 将你的变更以 PR 的形式提交过来，项目的维护人员会在第一时间对你的变更进行 review！
 1. 你也可以参考帮助文档 https://help.github.com/cn 了解更多细节。
 
 <p align="center">
-  <a href="https://github.com/doocs/leetcode"><img src="https://cdn.jsdelivr.net/gh/doocs/leetcode@main/images/how-to-contribute.png" alt="how-to-contribute"></a>
+  <a href="https://github.com/doocs/leetcode"><img src="https://cdn.jsdelivr.net/gh/doocs/leetcode@main/images/how-to-contribute.svg" alt="how-to-contribute"></a>
 </p>
 
 [Gitpod.io](https://www.gitpod.io) 是一个免费的在线开发环境，你也可以使用它参与本项目。

README_EN.md

@@ -109,12 +109,13 @@ Complete solutions to [LeetCode](https://leetcode.com/problemset/all/), [LCOF](h
 I'm looking for long-term contributors/partners to this repo! Send me [PRs](https://github.com/doocs/leetcode/pulls) if you're interested! See the following:
 
 1. Fork [this repository](https://github.com/doocs/leetcode) to your own GitHub account and then clone it to your local machine.
+1. Checkout a new branch.
 1. Make some changes to your leetcode repository, then push the changes to your remote GitHub repository.
 1. Create a pull request with your changes!
 1. See [CONTRIBUTING](https://github.com/doocs/.github/blob/main/CONTRIBUTING.md) or [GitHub Help](https://help.github.com/en) for more details.
 
 <p align="center">
-  <a href="https://github.com/doocs/leetcode"><img src="https://cdn.jsdelivr.net/gh/doocs/leetcode@main/images/how-to-contribute.png" alt="How to contribute"></a>
+  <a href="https://github.com/doocs/leetcode"><img src="https://cdn.jsdelivr.net/gh/doocs/leetcode@main/images/how-to-contribute.svg" alt="how-to-contribute"></a>
 </p>
 
 You can also contribute to [doocs/leetcode](https://github.com/doocs/leetcode) using [Gitpod.io](https://www.gitpod.io), a free online dev environment with a single click.