doc: explain why GitHub merge button is not used

Adds documentation and explicit reasons on why
the GitHub web interface button is not used. This was
explained in the referenced issue by @TheAlphaNerd.

Fixes: https://github.com/nodejs/node/issues/8893
PR-URL: https://github.com/nodejs/node/pull/9044
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/onboarding.md

@@ -164,8 +164,15 @@ onboarding session.
 
 ## Landing PRs: Details
 
-* Please never use GitHub's green "Merge Pull Request" button.
+* Please never use GitHub's green ["Merge Pull Request"](https://help.github.com/articles/merging-a-pull-request/#merging-a-pull-request-using-the-github-web-interface) button.
   * If you do, please force-push removing the merge.
+  * Reasons for not using the web interface button:
+    * The merge method will add an unnecessary merge commit.
+    * The rebase & merge method adds metadata to the commit title.
+    * The rebase method changes the author.
+    * The squash & merge method has been known to add metadata to the commit title.
+    * If more than one author has contributed to the PR, only the latest author will be considered during the squashing.
+
 
 Update your `master` branch (or whichever branch you are landing on, almost always `master`)