Add readme.txt which includes instructions how to build the jar and site.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/logging/trunk@1607019 13f79535-47bb-0310-9956-ffa450edef68
Add readme to source distribution.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/logging/trunk@1607021 13f79535-47bb-0310-9956-ffa450edef68