ZOOKEEPER-2880: Rename README.txt to README.md

- Change format of README.txt to markdown.

Author: Manoj Mallela <manojmallela@users.noreply.github.com>

Reviewers: Michael Han <hanm@apache.org>

Closes #341 from manojmallela/master

(cherry picked from commit 86577c9d81bd40067bc084e49a7172884ac9ae49)
Signed-off-by: Michael Han <hanm@apache.org>