"Actually Using Git": can't branch on an empty repo

On tutorials/actually-using-git/lessons/conflicting-branches.md , there are three reasonable ways to initialize a repo:
1. Reuse an existing repo
2. Create a new repo on Github and then clone it
3. Use `git init`

If a participant uses the third option (`git init`) and then, following the directions, immediately tries to create a new branch, they'll get an error:

```
fatal: Not a valid object name: 'master'.
```

Instead, they should create a dummy `readme.md`, add and commit it, and then proceed.

The instructions should be clarified for folks who choose the third option.

打开 tutorials/actually-using-git/lessons/conflicting-branches.md，并检查选择 git init 的参与者的说明。明确指出，他们应在创建分支前创建、添加并提交一个虚拟的 readme.md。完成的标准是 git init 路径不再导致所报告的错误。

由索引模型根据 Issue 内容生成。