Update guideline of squashing commits (#71394)

* do not squash commits

* point to pr-guide

CONTRIBUTING.md

@@ -63,7 +63,7 @@ Project maintainers will merge changes that improve the product significantly an
 
 Maintainers will not merge changes that have narrowly-defined benefits, due to compatibility risk. The .NET Core codebase is used by several Microsoft products (for example, ASP.NET Core, .NET Framework 4.x, Windows Universal Apps) to enable execution of managed code. Other companies are building products on top of .NET, too. We may revert changes if they are found to be breaking.
 
-Contributions must also satisfy the other published guidelines defined in this document.
+Contributions must also satisfy the other published guidelines defined in this document as well as in [pr-guide docs](docs/pr-guide.md).
 
 ### DOs and DON'Ts
 

docs/pr-guide.md

@@ -11,6 +11,7 @@ To merge pull requests, you must have write permissions in the repository. If yo
 * All changes should follow the existing code style. You can read more about different code styles at [docs/coding-guidelines](coding-guidelines/).
 * Use Draft pull requests for changes you are still working on but want early CI loop feedback. When you think your changes are ready for review, [change the status](https://help.github.com/en/github/collaborating-with-issues-and-pull-requests/changing-the-stage-of-a-pull-request) of your pull request.
 * Avoid rebasing your changes. If you are asked to make changes during the review process do them as a new commit.
+* To resolve merge conflicts, use "merge" instead of "rebase".
 
 ## Pull Request Ownership