Document howto create a release patch.

howto/code/release-bugfix.rst
release_notes/voltha_2.12.rst
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  o Detail branch creation attached to branch=voltha-2.12.
  o Add suggested for howto sanity check git attributes
    and sandbox files.

Change-Id: I4d0f63037e0ca2f324e561ad26e66b9a9dfec4df
2 files changed