Re #6720 Update stack-integration-test docs for lld usage

Author: mpilgrem

CONTRIBUTING.md

@@ -582,6 +582,10 @@ stack build --flag stack:integration-tests stack --exec "stack-integration-test
 To disable folders named after `<PATTERN1>` and `<PATTERN2>`
 It's especially useful when some tests are taking a while to complete.
 
+On Linux, the `stack-integration-test` executable uses the `lld` linker and
+expects it to be on the PATH. The integration tests complete significantly
+quicker with `lld` than with the `ld.bfd` linker.
+
 ## Continuous integration (CI)
 
 We use [GitHub Actions](https://docs.github.com/en/actions) to do CI on Stack.

tests/integration/README.md

@@ -30,6 +30,10 @@ Note that this command can take a _long_ time. It's also more thorough
 than the quick command given above, as it will run each test with a
 clean `STACK_ROOT`.
 
+On Linux, the `stack-integration-test` executable uses the `lld` linker and
+expects it to be on the PATH. The integration tests complete significantly
+quicker with `lld` than with the `ld.bfd` linker.
+
 ## Helper scripts
 
 There are two helper scripts in this directory. Note that these may