Land rapid7#4231, more explicit instructs on commit msgs

Explicitly reminds people to be wary of default (merge) messages from
Git when landing PRs on Metasploit.

Author: trevrosen

CONTRIBUTING.md

@@ -37,8 +37,10 @@ and Metasploit's [Common Coding Mistakes](https://github.com/rapid7/metasploit-f
 ## Code Contributions
 
 * **Do** stick to the [Ruby style guide](https://github.com/bbatsov/ruby-style-guide).
-* *Do* get [Rubocop](https://rubygems.org/search?query=rubocop) relatively quiet against the code you are adding or modifying.
+* **Do** get [Rubocop](https://rubygems.org/search?query=rubocop) relatively quiet against the code you are adding or modifying.
 * **Do** follow the [50/72 rule](http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html) for Git commit messages.
+* **Don't** use the default merge messages when merging from other
+   branches.
 * **Do** create a [topic branch](http://git-scm.com/book/en/Git-Branching-Branching-Workflows#Topic-Branches) to work on instead of working directly on `master`.
 
 ### Pull Requests