add bash completion to sdist

When installing from the source distribution (for packaging or other
purposes), it's convenient to be able to install the bash completion
file as part of that process.

MANIFEST.in

@@ -4,6 +4,7 @@ include requirements.txt
 include requirements-dev.txt
 include tox.ini
 include *.md
+include contrib/completion/bash/docker-compose
 recursive-include tests *
 global-exclude *.pyc
 global-exclude *.pyo