Update README.md to include :no_entry: deprecated note

[BenWinchester]

Description

This pull request is being opened to apply a note to the top of the README.md file in the CLOVER v4.0 repository to inform users that the version of CLOVER here is deprecated and that they should refer to the CLOVER-energy/CLOVER repository for up-to-date versions from v5.0 onwards 😄

[BenWinchester]

NOTE: As part of this review, several steps from the guide here should be added, namely:

• [ ] Change the GitHub description. This is often the first thing people see for the repository, and the first line that people will see if looking at the repository in the Organization view. The most important thing to do is to add the word DEPRECATED at the front of the description. This clearly marks it will not be updated. You can also add an emoji to it. ⛔️ DEPRECATED <former description> makes it pop out a bit more.
• [ ] Add GitHub topics: deprecated, obselete, and archived are all good ones.