Add BRANCH-README file.

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/issue-3390-dev@879976 13f79535-47bb-0310-9956-ffa450edef68

BRANCH-README

diff --git a/BRANCH-README b/BRANCH-README
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+This branch exists for the resolution of issue #3390.  It's intent is
+to be managed as a reintegrate-able branch, with regular catch-up
+merges from the trunk.
+
+Having said that, the state of svn:externals processing on the trunk
+has apparently diverged vastly since this branch was cut, and some are
+advising that the branch be trashed and re-cut from a newer trunk.