line 1 (should be empty for markdown, but for testing we only care numbers)
[make README.md]: # (include test_content.txt)

[source test_content.txt](test_content.txt)

```
some include test content
line 2

line 4
end
```

line 12

[make README.md]: # (echo "new outputed content")

```
new outputed content
```

some text after
line 23
