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

[kerber0x]

Description and Motivation

This PR adds information about the functioning of the pool manager to its readme file.

Related Issues

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Checklist:

• I have read Migaloo's contribution guidelines.
• My pull request has a sound title and description (not something vague like Update index.md)
• All existing and new tests are passing.
• 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]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.55%. Comparing base (0f77427) to head (235a0e6).
Report is 3 commits behind head on release/v2_contracts.

Additional details and impacted files

@@                   Coverage Diff                    @@
##           release/v2_contracts     #356      +/-   ##
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+ Coverage                 90.09%   90.55%   +0.45%     
========================================================
  Files                       278      276       -2     
  Lines                     29604    29921     +317     
========================================================
+ Hits                      26673    27094     +421     
+ Misses                     2931     2827     -104     

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[0xFable]

LGTM, we could mention more on routing and reuse more of router.md but im fine with it as is great work ser 🚀

[kerber0x]

LGTM, we could mention more on routing and reuse more of router.md but im fine with it as is great work ser 🚀

I in purpose omitted details about it cuz it's not critical to understand how the contract works and what you can do about it. We already mention that it can do single and multi-hop operations and what messages you need to use for them, i think that's enough. Then when we link to the technical docs there will be examples on how those messages should look like.