Version 1.6

• several typos and corrections
• corrected and simplified the section on gradient descent
• added a Python implementation of learning by gradient descent
• images are bitmapped for better compatibility with ebook readers

Version 1.5

more typos and corrections. Thanks to all those who spotted mistakes!

Version 1.4

more typos and corrections. Thanks to all those who spotted mistakes!

Version 1.3

typos fixed, some sentences clarified.

Version 1.2

more typos fixed

Version 1.1

fixed several spelling mistakes

Version 1.0

v1.0

Update README.md