make the build easier for people building from source

There are build details on the project README. People who want to build pekko-management can also look at the `.github/workflows` directory to get an idea of how the CI builds run.

All the same, with us having to produce source distributions for ASF release compliance and the idea the even git users might clone the repo and look to get the build done locally - we probably need to look at documenting more about the build.

Also, there is probably scope to make the build easier.

The integration tests are the hardest bit to run.

Start with the build details in the project README and compare them with the workflows in .github/workflows to understand how CI builds run. Investigate why the integration tests are hardest to run and determine what build instructions or improvements are needed. Done means the source-build process is documented more completely and the integration-test setup is easier to follow.

Written by the indexing model from the issue text.