Set maintenance mode

[mikesposito]

Description

This PR adds a warning on the main README, to inform package users and devs that EthKeyringController has been moved to the core monorepo, and merged into KeyringController.

Because of that, PRs will no longer be accepted, with the exception of security patches.

Changes

No functional changes

References

• Fixes https://github.com/MetaMask/core/issues/3768

Checklist

• [ ] I've updated the test suite for new or updated code as appropriate
• [ ] I've updated documentation for new or updated code as appropriate (note: this will usually be JSDoc)
• [ ] I've highlighted breaking changes using the "BREAKING" category above as appropriate