docs: add documentation for dep8 tests

dev-docs/howtoguides/run_dep8_tests.md

@@ -0,0 +1,27 @@
+# How to run dep8 tests
+
+The ubuntu-advantage-tools source package currently supports [dep8
+tests]( https://salsa.debian.org/ci-team/autopkgtest/-/blob/master/doc/README.package-tests.rst).
+Those dep8 tests can be found under `debian/tests/usage` and although they are currently only
+running a few Pro client commands, the idea is to verify if any modification to our package dependencies
+will affect the Pro client.
+
+Right now, we are looking for signal from our `python3-apt` dependency. That's why we are running
+the `packages` API there, as those endpoints directly interact with APT.
+
+If you perform any modification to those tests, you can verify it by following these steps:
+
+1. Install the `autopkgtest` application:
+
+```shell
+sudo apt install autopkgtest
+```
+
+2. Run the following command:
+
+```shell
+autopkgtest -U --shell-fail . -- lxd ubuntu:xenial
+```
+
+Note that you can run this command on any release we support, not only Xenial,
+and it will run for all releases by launchpad when dependencies are updated.