Updated developer guide after switch from rebasing to merging (#527)

Co-authored-by: Adrian Palacios <73246657+adpaco-aws@users.noreply.github.com>

Author: 2 people

DEVELOPER-GUIDE.md

@@ -13,8 +13,7 @@
    ```
 
 ## Submitting / updating PRs
-1. Ensure that your changes are rebased against the latest `main-<upstream-version>-yyyy-mm-dd`.
-   If your PR is open across a `main*` branch update, we recommend doing a rebase and force push.
+1. Ensure that your changes are rebased against `main`.
 1. Ensure that your code is properly formatted.
    ```
    ./x.py fmt
@@ -31,16 +30,4 @@ TODO
 RMC is implemented as an additional codegen backend for the 
 [Rust compiler](https://github.com/rust-lang/rust).
 The `master` branch from the upstream compiler is mirrored as `upstream-master`.
-The RMC code itself is maintained as a set of rebased patches, 
-    on branches named `main-<upstream-version>-yyyy-mm-dd`.
-These branches are updated on a weekly cadence.
-The most recent of these branches is set as the `default` branch for the repository.
-This is the branch that you should build and use, and this is the branch that you should make PRs against.
-
-### Patch structure
-The `main-<upstream-version>-yyyy-mm-dd` branches have the following git structure:
-
-* The upstream `master` branch as of the date `yyyy-mm-dd`.
-* A source code patch that makes all changes to the upstream code needed for RMC to link.
-* A renaming patch that renames upstream files that conflict with RMC files.
-* A set of commits representing RMC feature code.
+Changes from the upstream compiler are merged with RMC's `main` branch on a weekly cadence.