document inline variable usage on reference page

[dillongreen]

https://github.com/jessemiller/HamlPy/blob/master/reference.md should show an example for inline variable usage as otherwise folks would need to literally write

Hello
= user.name
,how are you today?

instead of just Hello #{user.name}, how are you today?

Inline variables were added about two years ago with #17

[jessemiller]

Fixed. You are welcome to do a pull request next time :)

[dillongreen]

Cheers, will do ;)