Update chapter 15 (полностью)
change format to markdown
Update chapter-20.md
change format to markdown
Merge pull request #33 from urtica/patch-3

Patch 3
Merge pull request #34 from urtica/patch-2

Patch 2
Правки по тексту Гл.16.
Merge pull request #35 from YaMike/master

Правки по тексту Гл.16.
Update chapter-16.md
Edit half of chapter 16