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

[kerber0x]

Description and Motivation

This PR adds information about how the incentive manager works on a high level on a readme file.

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 (61ed894) to head (04a91a8). Report is 1 commits behind head on release/v2_contracts.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## release/v2_contracts #354 +/- ## ===================================================== Coverage 91.15% 91.15% ===================================================== Files 279 279 Lines 31119 31119 ===================================================== Hits 28368 28368 Misses 2751 2751 ```

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