readme: Add compatibility section

The section provides more information about which Linux distributions
and versions are supported by Droplet Agent.

README.md

@@ -14,6 +14,7 @@ Droplet Agent is the daemon that runs on DigitalOcean's customer droplets to ena
   * [Packaging](#building-from-source-code)
 * [Running the Agent](#running-the-agent)
 * [Running Tests](#running-tests)
+* [Compatibility](#compatibility)
 * [Contributing](#contributing)
 
 ## Building
@@ -60,7 +61,6 @@ please remember to apply the changes by running `systemctl daemon-reload`
 7. Configuration for `initctl` is saved at `/etc/init/droplet-agent.conf`. If updated, please run
 `initctl reload-configuration` to apply the updated configuration.
 
-
 ## Running the Agent
 The agent binary takes several command line arguments:
 - `-debug` (boolean), if provided, the agent will run in debug mode with verbose logging. This is useful when debugging.
@@ -93,6 +93,15 @@ Then, inside the droplet-agent project directory:
 > make test
 ```
 
+## Compatibility
+
+Droplet Agent currently supports:
+
+- Ubuntu (oldest [End Of Standard Support](https://wiki.ubuntu.com/Releases) LTS release and later)
+- Debian ([oldest supported](https://wiki.debian.org/LTS) LTS release and later)
+- Fedora 27+
+- CentOS 6+
+
 ## Contributing
 
 Please read [CONTRIBUTING.md](CONTRIBUTING.md) for details on our code of conduct, and the process for submitting pull requests.