CONTRIBUTING: Remove branch-naming suggestions

Extending the earlier softening in 0548361 (CONTRIBUTING: Make
leader-issues optional, 2016-05-19, opencontainers#6), because nobody cares about
the XXX- prefix.  Checking the recent history of all OCI project
repos:

  $ for REPO in image-spec image-tools runc runtime-spec runtime-tools;
  > do
  >   (
  >     cd "${REPO} &&
  >     git fetch origin &&
  >     BRANCHES=$(git log --first-parent --oneline origin/master | sed 's/^.* from //') &&
  >     echo "${REPO} branches: $(echo "${BRANCHES}" | wc -l)" &&
  >     echo "${REPO} numbered: $(echo "${BRANCHES}" | grep '/[0-9]' | wc -l)"
  >   );
  > done
  image-spec branches: 276
  image-spec numbered: 1
  image-tools branches: 27
  image-tools numbered: 0
  runc branches: 1299
  runc numbered: 21
  runtime-spec branches: 391
  runtime-spec numbered: 4
  runtime-tools branches: 232
  runtime-tools numbered: 3

Signed-off-by: W. Trevor King <wking@tremily.us>

CONTRIBUTING.md

@@ -24,16 +24,9 @@ incorporating a new feature.
 
 ### Conventions
 
-Fork the repo and make changes on your fork in a feature branch:
-
-- If it's a bugfix branch, name it XXX-something where XXX is the number of the
-  issue
-- If it's a feature branch, create an enhancement issue to announce your
-  intentions, and name it XXX-something where XXX is the number of the issue.
-
-Small changes or changes that have been discussed on the project mailing list
-may be submitted without a leader issue, in which case you are free to name
-your branch however you like.
+Fork the repo and make changes on your fork in a feature branch.
+For larger bugs and enhancements, consider filing a leader issue or mailing-list thread for discussion that is independent of the implementation.
+Small changes or changes that have been discussed on the project mailing list may be submitted without a leader issue.
 
 If the project has a test suite, submit unit tests for your changes. Take a
 look at existing tests for inspiration. Run the full test suite on your branch