Improve README (#418)

* List missing plugins.

* Fix short description.

* Improve section on requirements.

* Apply suggestions from code review

Co-authored-by: Don Naro <dnaro@redhat.com>

Co-authored-by: Don Naro <dnaro@redhat.com>

README.md

@@ -16,15 +16,26 @@ Please note that Ansible 2.9 and ansible-base 2.10 are no longer supported. If y
 
 ## External requirements
 
-Most modules and plugins require the [Docker SDK for Python](https://pypi.org/project/docker/) and Python 2.7 or newer. Python 2.6 is no longer supported; use community.docker 2.x.y if you need to use Python 2.6.
+Some modules and plugins require Docker CLI, or other external, programs. Some require the [Docker SDK for Python](https://pypi.org/project/docker/) and some use [requests](https://pypi.org/project/requests/) to directly communicate with the Docker daemon API. All modules and plugins require Python 2.7 or later. Python 2.6 is no longer supported; use community.docker 2.x.y if you need to use Python 2.6.
 
-If you have Docker SDK for Python < 2.0.0 installed ([docker-py](https://pypi.org/project/docker-py/)), you can still use it, though we recommend to uninstall it and then install [docker](https://pypi.org/project/docker/), the Docker SDK for Python >= 2.0.0. Note that both libraries cannot be installed at the same time. If you accidentally did install them simultaneously, you have to uninstall *both* before re-installing one of them.
+Installing the Docker SDK for Python also installs the requirements for the modules and plugins that use `requests`. If you want to directly install the Python libraries instead of the SDK, you need the following ones:
+
+- [requests](https://pypi.org/project/requests/);
+- [pywin32](https://pypi.org/project/pywin32/) when using named pipes on Windows with the Windows 32 API;
+- [paramiko](https://pypi.org/project/paramiko/) when using SSH to connect to the Docker daemon with `use_ssh_client=false`;
+- [pyOpenSSL](https://pypi.org/project/pyOpenSSL/) when using TLS to connect to the Docker daemon;
+- [backports.ssl_match_hostname](https://pypi.org/project/backports.ssl_match_hostname/) when using TLS to connect to the Docker daemon on Python 2.
+
+If you have Docker SDK for Python < 2.0.0 installed ([docker-py](https://pypi.org/project/docker-py/)), you can still use it for modules that support it, though we recommend to uninstall it and then install [docker](https://pypi.org/project/docker/), the Docker SDK for Python >= 2.0.0. Note that both libraries cannot be installed at the same time. If you accidentally did install them simultaneously, you have to uninstall *both* before re-installing one of them.
 
 ## Included content
 
 * Connection plugins:
-  - community.docker.docker: use Docker containers as remotes
+  - community.docker.docker: use Docker containers as remotes using the Docker CLI program
+  - community.docker.docker_api: use Docker containers as remotes using the Docker API
+  - community.docker.nsenter: execute commands on the host running the controller container
 * Inventory plugins:
+  - community.docker.docker_containers: dynamic inventory plugin for Docker containers
   - community.docker.docker_machine: collect Docker machines as inventory
   - community.docker.docker_swarm: collect Docker Swarm nodes as inventory
 * Modules:

plugins/inventory/docker_containers.py

@@ -13,7 +13,7 @@ __metaclass__ = type
 
 DOCUMENTATION = '''
 name: docker_containers
-short_description: Ansible dynamic inventory plugin for Docker containers.
+short_description: Ansible dynamic inventory plugin for Docker containers
 version_added: 1.1.0
 author:
     - Felix Fontein (@felixfontein)