Clarify docs around g:goyo_linenr

Hello,

I am trying to use goyo.vim and I am confused regarding what `g:goyo_linenr` does. As far as I can tell a value of `0` turns off line numbers, while a value of anything else (-1, 1, 100, etc.) turns them on. Also, the `nr` at the end of `linenr` does not make it immediately apparent (at least to me) that the setting has to do with showing line numbers. I didn't even notice the `nr` and first began exploring the callbacks in order to display the line numbers.

If I can receive confirmation that I understand the variable/setting correctly, I'd like to submit a PR to update the docs to add additional information around what `g:goyo_linenr` does.

Thank you for this tool. 😄 ❤️