git basics demo: make it clear that we need to create hello.txt file.

[sblunt]

From zoom chat this year:

I’m working with the git_demo.md file from the CodeAstro repository, but I encounter an error when attempting to create hello.txt with the command: git add hello.txt

What’s the error? Does hello.txt already exist on your computer?

git add hello.txt
fatal: pathspec 'hello.txt' did not match any files

Ah yes. You need to make a file called hello.txt first!

Thanks! ☺️
The guide isn’t clear on this point. It requires that an archive already exists.