On the thunder branch: Add problem description and BRANCH-README.

* BRANCH-README
  (): New file.

* notes/thundering-herd.txt
  (): New file describing the problem and the attempted solution.


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/thunder@1586391 13f79535-47bb-0310-9956-ffa450edef68