Asterisk versions

[florisk]

We are currently using this library, along with the ari-proxy, on asterisk 16.9.0 without any issues. We are now considering an upgrade to asterisk 17 or 18.
It is not clear from the documentation which asterisk versions are supported, or tested with.
For instance the release notes for Asterisk 17.5.0 mentioned ARI changes that might be useful, but seems to be missing in the current code.

res_ari_channels
------------------
 * When creating a channel in ARI using the create call
   you can now specify dialplan variables to be set as part
   of the same operation.

It would be nice if the README could contain any Asterisk version compatibility information.

[Ulexus]

There definitely may be missing features, but I'm happy to add them (or accept PRs which add them). If they don't exist, it just means I haven't had cause to use them yet. As for existing features, we use this package up to the latest version of Asterisk (19 as of this writing).