Update `vcdiff-decoder` README post-v2

[lawrence-forooghian]

In ably-js v2 (after #1512 is merged) we bundle the https://github.com/ably-forks/vcdiff-decoder library with ably-js. That means that in v2, users will no longer need to directly interact with that library. So, once v2 is released, we should update the README in that library to make it clear that the instructions there only apply to ably-js v1.

┆Issue is synchronized with this Jira Task by Unito

[sync-by-unito[bot]]

➤ Automation for Jira commented:

The link to the corresponding Jira issue is https://ably.atlassian.net/browse/SDK-3951

[lawrence-forooghian]

No longer required, since in https://github.com/ably/ably-js/pull/1663 we restored the ClientOptions.plugins mechanism and the direct importing of vcdiff-decoder.