doc: adding "rules of thumb"

CONTRIBUTING.md

@@ -30,6 +30,48 @@ works.
 
 This document will guide you through the contribution process.
 
+### Rules of thumb
+
+<details>
+<summary><strong>Provide motivation for the change</strong></summary>
+Try to explain why this change will make the code better. For example, does
+it fix a bug, or is it a new feature, etc. This should be expressed in the
+commit messages as well as in the PR description.
+</details>
+<details>
+<summary><strong>Don't make <em>unnecessary</em> code changes</strong></summary>
+<em>Unnecessary</em> code changes are changes made because of personal preference
+or style. For example, renaming of variables or functions, adding or removing
+white spaces, and reordering lines or whole code blocks. These sort of
+changes should have a good reason since otherwise they cause unnecessary
+<a href="https://blog.gitprime.com/why-code-churn-matters">"code churn"</a>.
+As part of the project's strategy we maintain multiple release lines, code
+churn might hinder back-porting changes to other lines. Also when you
+change a line, your name will come up in `git blame` and shadow the name of
+the previous author of that line.
+</details>
+<details>
+<summary><strong>Keep the change-set self contained but at a reasonable size</strong></summary>
+Use good judgment when making a big change. If a reason doesn't come to mind
+but a very big change needs to be made, try to break it into smaller
+pieces (still as self-contained as possible), and cross-reference them,
+explicitly stating that they are dependent on each other.
+</details>
+<details>
+<summary><strong>Be aware of our style rules</strong></summary>
+As part of accepting a PR the changes <strong>must</strong> pass our linters.
+<ul>
+<li>For C++ we use Google's `cpplint` (with some adjustments) so following their
+<a href="https://github.com/google/styleguide">style-guide</a> should make your code
+compliant with our linter.</li>
+<li>For JS we use this
+<a hreaf="https://github.com/nodejs/node/blob/master/.eslintrc.yaml">rule-set</a>
+for ESLint plus some of
+<a href="https://github.com/nodejs/node/tree/master/tools/eslint-rules">our own custom rules</a>.</li>
+<li>For markdown we have a <a href="https://github.com/nodejs/node/blob/master/doc/STYLE_GUIDE.md">style guide</a></li>
+</ul>
+</details>
+
 ### Step 1: Fork
 
 Fork the project [on GitHub](https://github.com/nodejs/node) and check out your
@@ -122,6 +164,7 @@ changed and why. Follow these guidelines when writing one:
     Refs: http://eslint.org/docs/rules/space-in-parens.html
     Refs: https://github.com/nodejs/node/pull/3615
     ```
+    - If it is not a bug fix, document the motivation for the change.
 
 A good commit log can look something like this: