docs: Formalize our design docs process a bit.

This adds the basic outline of _when_ a Design Doc should be written. See the next
commit in the stack for the blueprint. 

By adding this we hopefully can prevent unnecessary churn from new and longtime contributors, 
when they want to add a major feature or rewrite a core part of Jujutsu. The text is written
as a guideline, not a rule.

docs/contributing.md

@@ -64,6 +64,12 @@ your company's interests, do feel free to approve it.
 This project follows [Google's Open Source Community
 Guidelines](https://opensource.google/conduct/).
 
+## Contributing large patches
+
+While we appreciate that you took the time to design/redesign or rework an 
+existing component, we cannot accept it as easily as that. As it requires a
+architecture review from multiple stakeholders, which we do with 
+[Design Docs](design_docs.md), see the [process here](design_docs.md#process). 
 
 ## Contributing to the documentation
 

docs/design_docs.md

@@ -0,0 +1,22 @@
+# Jujutsu Design Docs
+
+Jujutsu uses Design Docs to drive technical decisions on large projects and it
+is the place to discuss your proposed design or new component. It is a very 
+thorough process, in which all concerns must be addressed. It shares some
+similarities with [Rust RFCs] but mostly addresses _technical_ problems and 
+gauges the technical and social concerns of all stakeholders.
+
+So if you want to start building the native backend or the server component for
+Jujutsu, you'll need to go through this process. It also is good if you're 
+already familiar with the codebase, to not stall the implementation.
+
+## Process
+
+1. Add a new markdown document to `docs/design`, named after your improvement 
+   or project. 
+1. Describe the current state of the world and the things you want to improve.
+1. Wait for the Maintainers and Stakeholders to show up. 
+1. Iterate until everyone accepts the change in normal codereview fashion.
+
+
+[Rust RFCs]: https://github.com/rust-lang/rfcs 

mkdocs.yml

@@ -128,6 +128,7 @@ nav:
 - Contributing:
       - 'Guidelines and "How to...?"': 'contributing.md'
       - 'Code of conduct': 'code-of-conduct.md'
+      - 'Design Docs': 'design_docs.md'
 
 - 'Design docs': 
     - 'git-submodules': 'design/git-submodules.md'