In the top-level README.md, fix links to
other READMEs:

* adjust the reference to the wsd one to include the
  `.md` suffix as the file was renamed in commit
  79b667a3



* Use the path to the actual file for the URL/target,
  not just to the parent directory, as just browsing
  to the containing directory when clicking on the link
  text showing a specific file name is rather unexpected

Signed-off-by: Michael Weghorn <m.weghorn@posteo.de>
Change-Id: I81ceb48193bc5a7b00ceaedc6852696719fb90e6