doc: add note regarding commit message trailers

doc: add note regarding commit message trailers · nodejs/node@b514438

Original file line number	Diff line number	Diff line change
`@@ -184,6 +184,11 @@ A good commit message should describe what changed and why.`
`184`	`184`	of the log. Use the `Fixes:` prefix and the full issue URL. For other
`185`	`185`	references use `Refs:`.
`186`	`186`
	`187`	+`Fixes:` and `Refs:` trailers get automatically added to your commit message
	`188`	`+ when the Pull Request lands as long as they are included in the`
	`189`	`+ Pull Request's description. If the Pull Request lands in several commits,`
	`190`	`+ by default the trailers found in the description are added to each commits.`
	`191`	`+`
`187`	`192`	`Examples:`
`188`	`193`
`189`	`194`	* `Fixes: https://github.com/nodejs/node/issues/1337`