[server/README] Revert README and README.md as the original.

Originally README.md is a real file and README is a link to it.
But commit: 91d408b wrongly changed the link to a real file.
Then commit: 377bcac wrongly makes README.md a link and makes
README a real file in opposite.
Merge branch 'issue-171-5' of github.com:project-hatohol/hatohol into issue-171-5