dependabot.yml: document auto-merge in comments
dependabot.yml: document auto-merge in comments

It's frustrating that auto-merge is not supported and there is no
clear consensus for how, or whether, it should be supported.  Document
what I've found so far.

Signed-off-by: Kevin Locke <ffb4761cba839470133bee36aeb139f58d7dbaa9@kevinlocke.name>

kevinoid/stream-compare,kevinoid/node-project-template,kevinoid/promised-read,kevinoid/appveyor-status,kevinoid/travis-status,kevinoid/promised-read,kevinoid/node-project-template,kevinoid/swagger-spec-validator,kevinoid/promised-read,kevinoid/stream-compare,kevinoid/appveyor-status,kevinoid/appveyor-status,kevinoid/travis-status,kevinoid/swagger-spec-validator,kevinoid/git-branch-is,kevinoid/swagger-spec-validator,kevinoid/git-branch-is,kevinoid/git-branch-is,kevinoid/node-project-template,kevinoid/travis-status,kevinoid/stream-compare