Add tests for negative price limit and update readme.md

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.07% :warning:

Comparison is base (2af4570) 100.00% compared to head (2006b7e) 99.93%. Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #193 +/- ## =========================================== - Coverage 100.00% 99.93% -0.07% =========================================== Files 14 14 Lines 1581 1581 =========================================== - Hits 1581 1580 -1 - Misses 0 1 +1 ``` | [Files Changed](https://app.codecov.io/gh/jonasbkarlsson/ev_smart_charging/pull/193?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jonas+Karlsson) | Coverage Δ | | |---|---|---| | [custom\_components/ev\_smart\_charging/coordinator.py](https://app.codecov.io/gh/jonasbkarlsson/ev_smart_charging/pull/193?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jonas+Karlsson#diff-Y3VzdG9tX2NvbXBvbmVudHMvZXZfc21hcnRfY2hhcmdpbmcvY29vcmRpbmF0b3IucHk=) | `99.71% <100.00%> (-0.29%)` | :arrow_down: | | [...omponents/ev\_smart\_charging/helpers/coordinator.py](https://app.codecov.io/gh/jonasbkarlsson/ev_smart_charging/pull/193?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jonas+Karlsson#diff-Y3VzdG9tX2NvbXBvbmVudHMvZXZfc21hcnRfY2hhcmdpbmcvaGVscGVycy9jb29yZGluYXRvci5weQ==) | `100.00% <100.00%> (ø)` | | | [custom\_components/ev\_smart\_charging/number.py](https://app.codecov.io/gh/jonasbkarlsson/ev_smart_charging/pull/193?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jonas+Karlsson#diff-Y3VzdG9tX2NvbXBvbmVudHMvZXZfc21hcnRfY2hhcmdpbmcvbnVtYmVyLnB5) | `100.00% <100.00%> (ø)` | |

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