docs(dev-guide): add guideline for atomic commits to the developer guide

docs(dev-guide): put atomic commits at the beginning

Author: Josh Hsieh

doc/dev-guide/src/coding-standards.md

@@ -1,10 +1,14 @@
-
 # Coding standards
 
 Generally we just follow good sensible Rust practices, clippy and so forth.
 However there are some practices we've agreed on that are not machine-enforced;
 meeting those requirements in a PR will make it easier to merge.
 
+## Atomic commits
+
+We use atomic commits across the repo. Each commit should represent a single unit of change.
+You can read more about atomic commits [here](https://www.aleksandrhovhannisyan.com/blog/atomic-git-commits).
+
 ## Import grouping
 
 In each file the imports should be grouped into at most 4 groups in the