docs(generic): add doc formatting guidelines based off of pycodestyle

CONTRIBUTING.md

@@ -6,7 +6,7 @@ contributions. They include, but are not limited to:
 * Constructive feedback
 * [Questions about usage](#questions-about-usage)
 * [Bug reports / technical issues](#before-opening-bug-reportstechnical-issues)
-* Documentation changes
+* [Documentation changes](#documentation-changes)
 * Feature requests
 * [Pull requests](#filing-pull-requests)
 
@@ -52,6 +52,18 @@ in the issue tracker to denote fairly-well-scoped-out bugs or feature requests t
 can pick up and work on. If any of those labeled issues do not have enough information, please feel
 free to ask constructive questions. (This applies to any open issue.)
 
+## Documentation changes
+
+When changing the API documentation, here are some rules to keep in mind.
+
+* The first line:
+  * should end with a period
+  * should be in imperative mood (e.g., "Create" instead of "Creates")
+  * First line should not be the function's "signature"
+* The first word of the first line:
+  * should be properly capitalized
+  * should not be "This"
+
 ## Filing Pull Requests
 
 Here are some things to keep in mind as you file pull requests to fix bugs, add new features, etc.: