Add missing includes to step-17 and step-18

This commit fixes up a regression introduced with commit
  06a70bad3cbbcbb6abc51e12f29e3b5dfffb928a
that removed unnecessary includes of base/multithread_info.h. Both steps
need them for multithread_info.n_threads()
Merge pull request #26 from tamiko/fix_includes_in_steps

Matthias: This commit fixes up a regression introduced with commit
06a70ba
that removed unnecessary includes of base/multithread_info.h. Both steps
need them for multithread_info.n_threads()
Switched README to md format.

Implemented Matthias' suggestions.
Merge pull request #20 from luca-heltai/pr_switch_to_markdown

Switched README to md format.

Changed the README in such a way that a markdown syntax parser
recognizes code blocks, links and title. Moved it to README.md
Fix installation.

Well, it seems we forgot to update doc/CMakeLists.txt to install the new
README.md file...
Merge pull request #28 from tamiko/fix_installation

Fix installation.