Update free-programming-books.md

Added a link to "Models of Computation" by John E. Savage to the
"Theoretical Computer Science" category.
Fix cliffnotes markdown link
Merge pull request #436 from Stevo-S/patch-1

Update free-programming-books.md
Merge pull request #437 from MHM5000/master

R language & Enhancement
Merge pull request #438 from nesquena/patch-2

Fix cliffnotes markdown link
TODO