docs: add readme to cw-contract crate

[seanchen1991]

Closes: #1218

Description

Rendered

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PR author checklist:

• [ ] Added changelog entry, using unclog.
• [ ] Added tests.
• [ ] Linked to GitHub issue.
• [ ] Updated code comments and documentation (e.g., docs/).
• [ ] Tagged one reviewer who will be the one responsible for shepherding this PR.

Reviewer checklist:

• [ ] Reviewed Files changed in the GitHub PR explorer.
• [ ] Manually tested (in case integration/unit/mock tests are absent).

[codecov[bot]]

Codecov Report

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

Project coverage is 67.31%. Comparing base (4eba9e1) to head (376e9a9).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1219 +/- ## ======================================= Coverage 67.31% 67.31% ======================================= Files 234 234 Lines 23553 23553 ======================================= Hits 15855 15855 Misses 7698 7698 ```

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[rnbguy]

I added more context and explanation to the README.