add 'project' process guidlines for larger contributions

Author: RalfJung

src/tools/miri/CONTRIBUTING.md

@@ -8,10 +8,43 @@ find useful.
 Check out the issues on this GitHub repository for some ideas. In particular,
 look for the green `E-*` labels which mark issues that should be rather
 well-suited for onboarding. For more ideas or help with hacking on Miri, you can
-contact us (`oli-obk` and `RalfJ`) on the [Rust Zulip].
+contact us on the [Rust Zulip]. See the [Rust website](https://www.rust-lang.org/governance/teams/compiler#team-miri)
+for a list of Miri maintainers.
 
 [Rust Zulip]: https://rust-lang.zulipchat.com
 
+### Larger-scale contributions
+
+If you are thinking about making a larger-scale contribution -- in particular anything that needs
+more than can reasonably fit in a single PR to be feature-complete -- then please talk to us before
+writing significant amounts of code. Generally, we will ask that you follow a three-step "project"
+process for such contributions:
+
+1. Clearly define the **goal** of the project. This defines the scope of the project, i.e. which
+   part of which APIs should be supported. If this involves functions that expose a big API surface
+   with lots of flags, the project may want to support only a tiny subset of flags; that should be
+   documented. A good way to express the goal is with one or more test cases that Miri should be
+   able to successfully execute when the project is completed. It is a good idea to get feedback
+   from team members already at this stage to ensure that the project is reasonably scoped and
+   aligns with our interests.
+2. Make a **design** for how to realize the goal. A larger project will likely have to do global
+   changes to Miri, like adding new global state to the `Machine` type or new methods to the
+   `FileDescription` trait. Often we have to iterate on those changes, which can quite substantially
+   change how the final implementation looks like.
+
+    The design should be reasonably concrete, i.e. for new global state or methods the corresponding
+   Rust types and method signatures should be spelled out. We realize that it can be hard to make a
+   design without doing implementation work, in particular if you are not yet familiar with the
+   codebase. Doing draft implementations in phase 2 of this process is perfectly fine, just please
+   be aware that we might request fundamental changes that can require significantly reworking what
+   you already did. If you open a PR in this stage, please clearly indicate that this project is
+   still in the design stage.
+
+3. Finish the **implementation** and have it reviewed.
+
+This process is largely informal, and its primary goal is to more clearly communicate expectations.
+Please get in touch with us if you have any questions!
+
 ## Preparing the build environment
 
 Miri heavily relies on internal and unstable rustc interfaces to execute MIR,