Mention config docs to change log
As of April 2021, we use major version numbers (e.g. v2) to reflect backward incompatible changes to the workflow that likely require you to update your Nextstrain installation.
We also use this change log to document new features that maintain backward compatibility, indicating these features by the date they were added.
+## New features since last version update
+
+ - 12 May 2021: [Document all available workflow configuration parameters](https://nextstrain.github.io/ncov/configuration). ([#633](https://github.com/nextstrain/ncov/pull/633))
+
## v5 (7 May 2021)
[See the corresponding pull request](https://github.com/nextstrain/ncov/pull/615) for more details about this release.
