Added book to Machine Learning section

Textbook draft of Probabilistic Models in the Study of Language.
Update free-programming-books.md

I added some more Free Python sources and a link to PDF file.
Add a link to Mother Tongues of Computer Languages

Great poster!
Add Mother Tongues of Computer Languages Poster
added git section
Added Think Like (a) Git to the Git book list.
Added "wie werde ich unix guru"
Syntax fix
Merge pull request #181 from corragon/patch-1

Added book to Machine Learning section
Merge pull request #182 from techacker/patch-1

Update free-programming-books.md
Merge pull request #183 from kimsk/master

Add a link to Mother Tongues of Computer Languages Poster
Merge pull request #184 from samsky/patch-1

added git section and pro git book
Merge pull request #187 from fager/master

Added "Wie werde ich unix guru" to DE
Merge pull request #186 from jbfink/master

Added Think Like (a) Git to the Git book list.