"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.

Open tutorials/actually-using-git/lessons/conflicting-branches.md and review the instructions for participants who choose git init. Clarify that they should create, add, and commit a dummy readme.md before creating a branch. Done means the git init path no longer leads to the reported error.

Written by the indexing model from the issue text.