Add draft release info to opm release doc

[kevinrizza]

Update opm release docs with more explicit instructions about publishing automatically created draft releases generated by the github action.

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: kevinrizza

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [kevinrizza]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[codecov]

Codecov Report

Merging #670 (b6fbd12) into master (b660eab) will not change coverage.
The diff coverage is n/a.

❗ Current head b6fbd12 differs from pull request most recent head 45d6ea6. Consider uploading reports for the commit 45d6ea6 to get more accurate results

@@           Coverage Diff           @@
##           master     #670   +/-   ##
=======================================
  Coverage   48.71%   48.71%           
=======================================
  Files          88       88           
  Lines        7673     7673           
=======================================
  Hits         3738     3738           
  Misses       3200     3200           
  Partials      735      735           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b660eab...45d6ea6. Read the comment docs.

[timflannagan]

/lgtm