Docs: Updated CONTRIBUTING.md to provide more details about the PR process

Closes #6323

Author: clintongormley

CONTRIBUTING.md

@@ -42,7 +42,7 @@ The process for contributing to any of the [Elasticsearch repositories](https://
 
 ### Fork and clone the repository
 
-You will need to fork the main Elasticsearch code or documentation repository and clone it to your local machine. See 
+You will need to fork the main Elasticsearch code or documentation repository and clone it to your local machine. See
 [github help page](https://help.github.com/articles/fork-a-repo) for help.
 
 Further instructions for specific projects are given below.
@@ -62,15 +62,15 @@ Once your changes and tests are ready to submit for review:
 
 3. Rebase your changes
 
-    Update your local repository with the most recent code from the main Elasticsearch repository, and rebase your branch on top of the latest master branch. We prefer your changes to be squashed into a single commit.
+    Update your local repository with the most recent code from the main Elasticsearch repository, and rebase your branch on top of the latest master branch. We prefer your initial changes to be squashed into a single commit. Later, if we ask you to make changes, add them as separate commits.  This makes them easier to review.  As a final step before merging we will either ask you to squash all commits yourself or we'll do it for you.
+
 
 4. Submit a pull request
 
-    Push your local changes to your forked copy of the repository and [submit a pull request](https://help.github.com/articles/using-pull-requests). In the pull request, describe what your changes do and mention the number of the issue where discussion has taken place, eg "Closes #123".
+    Push your local changes to your forked copy of the repository and [submit a pull request](https://help.github.com/articles/using-pull-requests). In the pull request, choose a title which sums up the changes that you have made, and in the body provide more details about what your changes do. Also mention the number of the issue where discussion has taken place, eg "Closes #123".
 
 Then sit back and wait. There will probably be discussion about the pull request and, if any changes are needed, we would love to work with you to get your pull request merged into Elasticsearch.
 
-
 Contributing to the Elasticsearch codebase
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