Document new release branching process

We want to avoid publishing nighlies with a misleading version number. When we
promote master to beta, all subsequent commits on master should have an
incremented version number, but this is currently not quite true, as during
release week we first promote and then later bump version numbers.

Author: Mark-Simulacrum

src/release/process.md

@@ -2,6 +2,15 @@
 
 Here's how Rust is currently released:
 
+## Promote beta to stable (T-6 days, Friday the week before)
+
+Open a PR bumping the version number in `src/bootstrap/channel.rs`. r+
+rollup=never this PR.
+
+Mark it as rollup=never, because if it lands in a rollup as *not* the first PR
+then other pull requests in that rollup will be incorrectly associated with the
+prior release.
+
 ## Promote beta to stable (T-3 days, Monday)
 
 Promote beta to stable. Temporarily turn off GitHub branch protection for the
@@ -50,6 +59,14 @@ docker exec -d -it rcs bash -c 'promote-release /tmp/stable stable /data/secrets
 
 ## Promote master to beta (T-2 days, Tuesday)
 
+We need to find out the parent commit in which the PR opened last Monday merged.
+
+Go to that PR, and find the "bors merged commit $SHA into rust-lang:master at the bottom.
+
+Locally, run `git log $SHA --author=bors`, and take the SHA of the first
+bors-authored merge into master (should be the the first commit). Call that
+`$BRANCH_POINT`.
+
 Create a new branch on `rust-lang/cargo` for the new beta. Here, `rust-lang` is
 the remote for https://github.com/rust-lang/rust.git. Replace `YY` with the
 minor version of master. First determine the branch point for cargo in
@@ -58,7 +75,7 @@ minor version of master. First determine the branch point for cargo in
 ```sh
 $ cd rust
 $ git fetch rust-lang
-$ CARGO_SHA=`git rev-parse rust-lang/master:src/tools/cargo`
+$ CARGO_SHA=`git rev-parse $BRANCH_POINT:src/tools/cargo`
 $ cd src/tools/cargo
 $ git branch rust-1.YY.0 $CARGO_SHA
 $ git push origin rust-1.YY.0
@@ -75,7 +92,7 @@ repo. Promote rust-lang/rust's master branch to beta as with yesterday:
 
 ```sh
 $ git fetch rust-lang
-$ git push rust-lang rust-lang/master:beta -f
+$ git push rust-lang $BRANCH_POINT:beta -f
 ```
 
 Re-enable branch protection on GitHub. Send a PR to the freshly created beta
@@ -103,7 +120,8 @@ tomorrow.
 Send a PR to the master branch to:
 
 - modify src/stage0.txt to bootstrap from yesterday's beta
-- modify src/bootstrap/channel.rs with the new version number
+- Remove `cfg(stage0)` annotated items
+- Replace `cfg(not(stage0))` with nothing
 
 ## Release day (Thursday)