add awk script

Author: rickeylev

RELEASING.md

@@ -14,6 +14,9 @@ These are the steps for a regularly scheduled release from HEAD.
 
 1. [Determine the next semantic version number](#determining-semantic-version).
 1. Update CHANGELOG.md: replace the `v0-0-0` and `0.0.0` with `X.Y.0`.
+   ```
+   awk -v version=X.Y.0 'BEGIN { hv=version; gsub(/\./, "-", hv) } /END_UNRELEASED_TEMPLATE/ { found_marker = 1 } found_marker { gsub(/v0-0-0/, hv, $0); gsub(/Unreleased/, "[" version "] - " strftime("%Y-%m-%d"), $0); gsub(/0.0.0/, version, $0); } { print } ' CHANGELOG.md > /tmp/changelog && cp /tmp/changelog CHANGELOG.md
+   ```
 1. Replace `VERSION_NEXT_*` strings with `X.Y.0`.
    ```
    grep -l --exclude=CONTRIBUTING.md --exclude=RELEASING.md --exclude-dir=.* VERSION_NEXT_ -r \