.ci: add README.md.

.ci/README.md

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+# `.ci`
+
+This directory contains scripts for Travis CI and (more or less) Azure
+Pipelines, but they will also happily run on any Debian-like machine.
+
+The scripts are usually split into `_install` and `_test` steps. The `_install`
+step will damage your machine, the `_test` step will just run the tests the way
+CI runs them.
+
+There is a common library, `ci_lib.py`, which just centralized a bunch of
+random macros and also environment parsing.
+
+Some of the scripts allow you to pass extra flags through to the component
+under test, e.g. `../../.ci/ansible_tests.py -vvv` will run with verbose.
+
+Hack these scripts until your heart is content. There is no pride to be found
+here, just necessity.
+
+### Environment Variables
+
+* `VER`: Ansible version the `_install` script should install. Default changes
+  over time.
+* `TARGET_COUNT`: number of targets for `debops_` run. Defaults to 2.
+* `DISTRO`: the `mitogen_` tests need a target Docker container distro. This
+  name comes from the Docker Hub `mitogen` user, i.e. `mitogen/$DISTRO-test`
+* `DISTROS`: the `ansible_` tests can run against multiple targets
+  simultaneously, which speeds things up. This is a space-separated list of
+  DISTRO names, but additionally, supports:
+    * `debian-py3`: when generating Ansible inventory file, set
+      `ansible_python_interpreter` to `python3`, i.e. run a test where the
+      target interpreter is Python 3.
+    * `debian*16`: generate 16 Docker containers running Debian. Also works
+      with -py3.
+