Merge pull request #1762 from cortexproject/contrib-instructions

Add some words about what makes a good PR

Author: bboreham

CONTRIBUTING.md

@@ -6,6 +6,15 @@ Welcome! We're excited that you're interested in contributing. Below are some ba
 
 Cortex follows a standard GitHub pull request workflow. If you're unfamiliar with this workflow, read the very helpful [Understanding the GitHub flow](https://guides.github.com/introduction/flow/) guide from GitHub.
 
+You are welcome to create draft PRs at any stage of readiness - this
+can be helpful to ask for assistance or to develop an idea. But before
+a piece of work is finished it should:
+
+* Be organised into one or more commits, each of which has a commit message that describes all changes made in that commit ('why' more than 'what' - we can read the diffs to see the code that changed).
+* Each commit should build towards the whole - don't leave in back-tracks and mistakes that you later corrected.
+* Have tests for new functionality or tests that would have caught the bug being fixed.
+* Include a CHANGELOG message if users of Cortex need to hear about what you did.
+
 ## Developer Certificates of Origin (DCOs)
 
 Before submitting your work in a pull request, make sure that *all* commits are signed off with a **Developer Certificate of Origin** (DCO). Here's an example: