PROTON-342: Added a note to README about using DESTDIR.

Also includes a link to explain how its meant to be used.

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/trunk@1528112 13f79535-47bb-0310-9956-ffa450edef68
PROTON-342: Updated the README with text from gsims re: DESTDIR.

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/trunk@1528152 13f79535-47bb-0310-9956-ffa450edef68