[doc] RELEASING: Add example command to generate the release note

[cbentejac]

Description

This PR adds an example of the command to use to generate the release note.

./github-generate-release-note.py -o alicevision -r AliceVision -m MAJOR.MINOR.PATCH --highlights major-feature --label-include scope:ci,scope:doc,scope:build sensordb -s updated-asc

generates a list of all the closed pull requests for the milestone MAJOR.MINOR.PATCH with:

• a "Main features" section (highlights) containing all the PRs tagged with the label major-feature;
• an "Other improvements" section that contains all the PRs that are not tagged with the labels used in label-include;
• two sections (label-include) containing respectively:
  • all the PRs tagged with labels scope:ci, scope:doc, scope:build;
  • all the PRs tagged with the label sensordb.

All the pull requests are sorted with the "least recently updated" setting.