Clarify the major version bumps in CHANGELOGs

[blamario]

I didn't look closely into the actual commit history, but y'all seem to be trigger-happy with the major version bumps. Looking at just the changelogs, there's stuff like

## 2.0 -- 2021-11-11

- Plutus upgraded
  - `plutus` pinned to `3f089ccf0ca746b399c99afe51e063b0640af547`
  - `plutus-apps` pinned to `404af7ac3e27ebcb218c05f79d9a70ca966407c9`

How does a Plutus upgrade justify a super-major version bump? Maybe there was another reason for it, but in that case please state what it was so your downstream users can figure out what they need to change. And if there was no reason for it, then please refrain from major version bumps. They impose real costs on your users.

[kozross]

This is a policy established by @Benjmhart: specifically, in addition to the PVP, a Plutus bump imposes a major version jump. I agree, this should be documented.