README not linking to Muffin project properly

[Ilya Semenov]

Muffin-Admin -- an extension to Muffin_ that implements admin-interfaces

Muffin_ here is an invalid link that leads to a non-existent element (<a href="#id15">Muffin_</a>) rather than, expectedly, https://github.com/klen/muffin. Also, placing a period at the end of the sentence would be great :)

[Kirill Klenov]

Thank you for the PR. Going to fix it.