docs(epoch-manager): add readme to epoch manager

[kerber0x]

Description and Motivation

This PR adds a readme file explaining how the epoch manager works.

Related Issues

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Checklist:

• [x] I have read Migaloo's contribution guidelines.
• [x] My pull request has a sound title and description (not something vague like Update index.md)
• [ ] All existing and new tests are passing.
• [x] I updated/added relevant documentation.
• [ ] The code is formatted properly cargo fmt --all --.
• [ ] Clippy doesn't report any issues cargo clippy -- -D warnings.
• [ ] I have regenerated the schemas if needed cargo schema.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.15%. Comparing base (0f77427) to head (1d6ffbc). Report is 32 commits behind head on release/v2_contracts.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## release/v2_contracts #353 +/- ## ======================================================== + Coverage 90.09% 91.15% +1.06% ======================================================== Files 278 279 +1 Lines 29604 31119 +1515 ======================================================== + Hits 26673 28368 +1695 + Misses 2931 2751 -180 ```

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[kerber0x]

LGTM good work, I'd love to see some more here about how we are using the epoch manager at a high level i.e the clock and how that affects other contracts

Cool, will add that.