Cover common contributor mistakes in CONTRIBUTING.md

Based on https://github.com/twbs/rorschach/tree/master/docs

CONTRIBUTING.md

@@ -116,6 +116,17 @@ Please adhere to the [coding guidelines](#code-guidelines) used throughout the
 project (indentation, accurate comments, etc.) and any other requirements
 (such as test coverage).
 
+**Do not edit `bootstrap.css`, `bootstrap-theme.css`, or `bootstrap.js`
+directly!** Those files are automatically generated. You should edit the
+source files in [`/bootstrap/less/`](https://github.com/twbs/bootstrap/tree/master/less)
+and/or [`/bootstrap/js/`](https://github.com/twbs/bootstrap/tree/master/js) instead.
+
+Similarly, when contributing to Bootstrap's documentation, you should edit the
+documentation source files in
+[the `/bootstrap/docs/` directory of the `master` branch](https://github.com/twbs/bootstrap/tree/master/docs).
+**Do not edit the `gh-pages` branch.** That branch is generated from the
+documentation source files and is managed separately by the Bootstrap Core Team.
+
 Adhering to the following process is the best way to get your work
 included in the project: