docs: document design process

CONTRIBUTING.md

@@ -40,13 +40,30 @@ hot-reloading ftl agent:
 $ ftl dev ./examples/go
 ```
 
-## Development workflow
+## Development processes
 
-Because we're a widely distributed team, we use a review-after-merge development flow. That is, if a PR is urgent, minor, or the developer has high confidence, we encourage merging without waiting for review in order to decrease friction. Conversely, if a change is more complex, or needs more eyes, we encourage developers to wait for review if it will make them feel more comfortable. Use your best judgement.
+### Code reviews
+
+Because we're a geographically distributed team, we use a review-after-merge development flow. That is, if a PR is urgent, minor, or the developer has high confidence, we encourage merging without waiting for review in order to decrease friction. Conversely, if a change is more complex, or needs more eyes, we encourage developers to wait for review if it will make them feel more comfortable. Use your best judgement.
 
 We discourage bike-shedding. Code and documentation is easy to change, we can always adjust it later.
 
-## Best practices
+### Design process
+
+Many changes are relatively straightforward and don't require a design, but more complex changes should be covered by a design document. Once a DRI (Directly Responsible Individual, a Cash term) is selected, our process for creating and reviewing designs is the following. The DRI:
+
+1. Does some initial thinking and comes up with a rough approach.
+2. Schedules a short (30-60 minute) design session with the rest of the team to do a basic check that the idea is sane, and pick up any obvious issues.
+   - Not everyone needs to attend, but those who can should.
+   - Focus on the larger points, minor issues will naturally fall out during implementation.
+   - Avoid bikeshedding.
+3. Writes the design doc.
+4. Send the doc to the team for async review via HackMD comments.
+5. Wait for @alecthomas to approve, then implement!
+
+Our design docs are [stored in HackMD](https://hackmd.io/team/ftl). There's a template for design docs that should be used. Add a label to your design document, representing its state.
+
+## Coding best practices
 
 ### Optional