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

[Nikita Popov via llvm-commits]

================
@@ -176,6 +176,24 @@ request will understand that you're rebasing just your patches, and display
 this result correctly with a note that a force push did occur.
 
 
+Pre-merge Continuous Integration (CI)
+-------------------------------------
+
+Multiple checks will be applies on a pull-request, either for linting/formatting
+or some build and tests. None of these are perfect and you will encounter
+false positive, infrastructure failures (unstable or unavailable worker), or
+you will be unlucky and based your change on a broken revision of the main branch.
+
+None of the checks are strictly mandatory: these are tools to help us build a
+better codebase and be more productive (by avoiding issues found post-merge and
+possible reverts). As a developer you're empowered to exercise your judgement
+about bypassing any of the checks when merging code.
+
+The infrastructure can print message that make it seem like these are mandatory,
----------------
nikic wrote:

```suggestion
The infrastructure can print messages that make it seem like these are mandatory,
```
or "a message".

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