Philip issues

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                                            Philip

consider adding a bit more more info about cats implementation of Monoid[Option[Int]] ?

https://twitter.com/philip_schwarz/status/946863214897266691 ![image](https://user-images.githubusercontent.com/116804/34461752-6d2116c8-ee33-11e7-80ec-19dd033e8fef.png) ![image](https://user-images.githubusercontent.com/116804/34461754-745681e4-ee33-11e7-896b-db2aba3150fa.png) ![image](https://user-images.githubusercontent.com/116804/34461755-7a0fc2f8-ee33-11e7-92b0-8d75252e045e.png) ![image](https://user-images.githubusercontent.com/116804/34461758-80b91ff0-ee33-11e7-8797-caae9a97359a.png) ![image](https://user-images.githubusercontent.com/116804/34461759-8787950a-ee33-11e7-82e7-7c38f2684bdf.png) ![image](https://user-images.githubusercontent.com/116804/34461760-8df858ac-ee33-11e7-8cac-bad57f07edf7.png) ![image](https://user-images.githubusercontent.com/116804/34461761-938e2eb8-ee33-11e7-9bde-5860b1fadd90.png)

opportunity to mention 'init' function

Hello Sergei, When the book [defines](https://github.com/winitzki/sofp/blob/master/sofp-src/tex/sofp-induction.tex#L1353) `digitsToInt`, there could be an opportunity, either to replace `s.take(s.length - 1)` with `s.init`, or, if using `s.take(s.length - 1)` serves an essential purpose...

Correct overview link in README.md

The current link leads to a page that displays the following error message: > Page Not Found > The requested page could not be found. I am not claiming that...