Document Dependabot merge process (#4556)

Co-authored-by: Richard North <[email protected]>

Author: kiview

docs/contributing.md

@@ -90,3 +90,17 @@ New modules should have the following warning at the top of their documentation
     This module is INCUBATING. While it is ready for use and operational in the current version of Testcontainers, it is possible that it may receive breaking changes in the future. See [our contributing guidelines](/contributing/#incubating-modules) for more information on our incubating modules policy.
 
 We will evaluate incubating modules periodically, and remove the label when appropriate.
+
+
+## Combining Dependabot PRs
+
+Since we generally get a lot of Dependabot PRs, we regularly combine them into single commits. 
+For this, we are using the [gh-combine-prs](https://github.com/rnorth/gh-combine-prs) extension for [GitHub CLI](https://cli.github.com/).
+
+The whole process is as follow:
+
+1. Check that all open Dependabot PRs did succeed their build. If they did not succeed, trigger a rerun if the cause were external factors or else document the reason if obvious.
+2. Run the extension from an up-to-date local `master` branch: `gh combine-prs --query "author:app/dependabot"`
+3. Merge conflicts might appear. Just ignore them, we will get those PRs in a future run.
+4. Once the build of the combined PR did succeed, temporarily enable merge commits and merge the PR using a merge commit through the GitHub UI.
+5. After the merge, disable merge commits again.