[llvm] [Doc] Add a section on CI to the GitHub documentation (PR #85376)

[Nikita Popov via llvm-commits]

================
@@ -124,12 +124,23 @@ For developers to commit changes from Git
 -----------------------------------------
 
 Once a patch is reviewed, you can select the "Squash and merge" button in the
-GitHub web interface. You might need to rebase your change before pushing
-it to the repo.
+GitHub web interface.
 
-LLVM currently has a linear-history policy, which means that merge commits are
-not allowed. The `llvm-project` repo on github is configured to reject pushes
-that include merges, so the `git rebase` step above is required.
+When pushing directly from the command-line to the `main`` branch, you will need
+to rebase your change. LLVM currently has a linear-history policy, which means
+that merge commits are not allowed and the `main` branch is configured to reject
+pushes that include merges.
+GitHub will display a message that looks like:
+
+.. code-block:: console
+
+  remote: Bypassed rule violations for refs/heads/main:
+  remote:
+  remote: - Required status check “buildkite/github-pull-requests” is expected.
+
+This can seem scary, but you likely did nothing wrong: it is intended as a warning
+for people merging pull-request with failing CI. We can't disable it for people
----------------
nikic wrote:

```suggestion
for people merging pull-requests with failing CI. We can't disable it for people
```

https://github.com/llvm/llvm-project/pull/85376