Build Instructions wrt VERSION

The README.md [instructions](https://github.com/aws/session-manager-plugin#working-with-docker) seem to be incomplete. Following those instructions results in a binary that reports its version as `1.2.0.0`. This is true even if you checkout a (recent) specific version tag and build from that.

This wouldn't be an issue, but the version number it reports is load bearing. If the version is wrong, then this issue regarding timeouts re-emerges https://github.com/aws/aws-sdk-go/issues/4691. Editing the VERSION file directly and rerunning the build results in a binary without the issue.

I wasn't able to find where in the build tooling this VERSION file was intended to be updated to the correct version. I used `printf 1.2.463.0 > VERSION` to update the file to the latest release tag. It would be very helpful to have some instructions in the README.md on the proper way to update this file as part of the local build instructions.