Update README and CHANGELOG

Author: hush-hush

CHANGELOG.md

@@ -4,6 +4,7 @@ CHANGELOG
 # 2.2.0 / 2017-12-27
 - [FEATURE] Set log level to warning for the datadog and request packages. See [#24][] (thanks to @n0ts)
 - [FEATURE] Allow users to set a custom location for the configuration file.
+- [FEATURE] Added environment variables for configuring datadog api key. See [#22][] (thanks to @pyconsult)
 
 # 2.1.0 / 2017-12-26
 - [FEATURE] Disable callback if required python packages aren't installed. See [#28][] (thanks to @dobber)

README.md

@@ -21,7 +21,16 @@ Once the required libraries (see above) have been installed on the server:
 1. Copy `datadog_callback.py` to your playbook callback directory (by default
 `callback_plugins/` in your playbook's root directory). Create the directory
 if it doesn't exist.
-2. Create a `datadog_callback.yml` file alongside `datadog_callback.py`,
+2. You have 3 ways to set your API key. The callback will first use the
+   environment variable, then the configuration file, then hostvars/vault.
+
+##### Using environment variable
+
+Set the environment variable `DATADOG_API_KEY`.
+
+##### Using a yaml file
+
+Create a `datadog_callback.yml` file alongside `datadog_callback.py`,
 and set its contents with your [API key](https://app.datadoghq.com/account/settings#api),
 as following:
 
@@ -37,6 +46,8 @@ For exemple:
 ANSIBLE_DATADOG_CALLBACK_CONF_FILE=/etc/datadog/callback_conf.yaml ansible-playbook ...
 ```
 
+##### Using ansible hostvars and vault
+
 Alternatively you can use the hostvars of the host ansible is being run from (preferably in the vault file):
 ```
 datadog_api_key: <your-api-key>