Merge pull request #1288 from miguelarauj1o/scheme-link

Added scheme link in free-programming-books.md file

free-programming-books.md

@@ -1672,6 +1672,7 @@ See also [TeX](#tex)
 * [An Introduction to Scheme and its Implementation](http://www.cs.rpi.edu/academics/courses/fall00/ai/scheme/reference/schintro-v14/schintro_toc.html)
 * [Concrete Abstractions: An Introduction to Computer Science Using Scheme](https://gustavus.edu/+max/concrete-abstractions.html)
 * [How to Design Programs](http://htdp.org)
+* [Scheme Tutorial](http://www.cs.hut.fi/Studies/T-93.210/schemetutorial/)
 * [Simply Scheme: Introducing Computer Science](http://www.cs.berkeley.edu/~bh/ss-toc2.html)
 * [Structure and Interpretion of Computer Programs](http://mitpress.mit.edu/sicp/full-text/book/book.html)
 * [Teach Yourself Scheme in Fixnum Days](http://www.ccs.neu.edu/home/dorai/t-y-scheme/t-y-scheme.html)