doc: add README to dmclock subdir to inform developers it's a git subtree

Since dmclock was integrated into ceph a short time ago, twice now
contributors have submitted PRs to modify files below src/dmclock when
they should have modified the external dmclock library itself. This
README file is designed to hopefully warn future contributors and
instruct them on how best to approach changes.

Signed-off-by: J. Eric Ivancich <ivancich@redhat.com>

diff --git a/src/dmclock/README.before-modifying-files-here-or-below b/src/dmclock/README.before-modifying-files-here-or-below
new file mode 100644 (file)
index 0000000..2226512
--- /dev/null
+++ b/src/dmclock/README.before-modifying-files-here-or-below
@@ -0,0 +1,29 @@
+The dmclock is an EXTERNAL library is not wholly owned by the ceph
+repo.  Instead, it is brought into ceph repo via a "git subtree".
+
+If you are unfamiliar with this technique, PLEASE READ
+../../README.git-subtree.
+
+IT'S VERY LIKELY YOU DO *NOT* WANT TO DIRECTLY MODIFY FILES IN OR
+BELOW src/dmclock, AND THAT IF YOU DO MODIFY SUCH FILES YOUR
+MODIFICATIONS WILL LIKELY BE REJECTED.
+
+You should only modify files below src/dmclock if:
+
+    1) you intend to diverge the ceph version of the dmclock library
+       from the external library, and
+
+    2) you intend for ceph developers to maintain those divergences
+       for the foreseeable future, even as the dmclock library evolves
+       and those changes are pulled into ceph.
+
+[Note: this file is an example of such a modification; this file is
+such a divergence.]
+
+If you would like to submit a PR to the dmclock library itself, then
+you'll want to submit a PR to:
+
+    https://github.com/ceph/dmclock
+
+After that PR is merged, you would then submit a PR to ceph that does
+a "git subtree pull" (see ../../README.git-subtree for details).