Update CONTRIBUTING.md to reflect "Squash and merge" option

Currently, PR doesn't need to go through internal landing. Reviewers and contributors with "write permission" can just squash and merge and PR themselves.

Update the docs to reflect it.

Author: kirklandsign

CONTRIBUTING.md

@@ -272,8 +272,13 @@ for basics.
    - If the reviewers have requests or questions, follow up with them.
    - The goal of the reviewer is to ensure that the code in the `main` branch of
      the repo is consistent, maintainable, and of high quality.
-1. Once approved, your reviewer will import the PR into Meta's internal system
-   and merge it from there.
+1. Once approved, your reviewer will merge the PR.
+   - If you have the "write permission" to repo, and see the "Squash and merge"
+     button is green and all CI signals are good, you can merge it yourself. The
+     commit message in the text box when you click "Squash and merge" is the final
+     commit message in repo, so please make it meaningful.
+   - The reviewer may import the PR into Meta's internal system to validate it
+     against internal CI.
    - If the PR is approved and not merged within a few business days, please
      comment on the PR to ask about its status.
    - Note that if the `main` [CI](#continuous-integration) jobs are broken, we