Clarify docs around g:goyo_linenr

[aegatlin]

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. 😄 ❤️