Add cross-region inference considerations to README

[inariku]

Issue #, if available:
#14

Description of changes:

This PR adds important information about using Bedrock models with cross-region inference to the README. When deploying to regions requiring cross-region inference (marked with * in Supported Regions and models for inference profiles), users need to use inference profile IDs instead of direct model IDs.

The changes include:

1. Added a "Cross-Region Inference Considerations" subsection under Configuration
2. Provided guidance on how to check which regions require cross-region inference
3. Included an example configuration for regions requiring cross-region inference
4. Added troubleshooting tip for the common error message "Invocation of model ID with on-demand throughput isn't supported"

This documentation update helps users avoid common errors when deploying to regions that require cross-region inference, as the requirements may change as AWS updates their service offerings.

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