docs: Add clone and build steps to contributing guide

CONTRIBUTING.md

@@ -76,6 +76,27 @@ Here's the recommended workflow:
 If what you are going to work on is a substantial change, please first
 ask the core team for their opinion on the [RabbitMQ users mailing list][rmq-users].
 
+### Building Source
+
+To build the projects first, clone the repository. Make sure you clone the
+repository and its submodules:
+
+It is good practice to make sure you can build the project before making any
+changes to confirm that your development environment is set
+up correctly. Verifying that the tests pass is also a good practice (see
+[RUNNING_TESTS.md](/RUNNING_TESTS.md)).
+
+All together, this looks like:
+
+```shell
+git clone --recurse-submodules https://github.com/rabbitmq/rabbitmq-dotnet-client
+cd rabbitmq-dotnet-client
+dotnet build ./Build.csproj
+./.ci/ubuntu/gha-setup.sh   # On any Linux distribution with Docker installed
+./.ci/windows/gha-setup.ps1 # On Windows
+make test
+```
+
 ### Running Tests
 
 See [RUNNING_TESTS.md](/RUNNING_TESTS.md).