updated the readme for issue #4

[mukeshbharath]

Hello Terry, I have updated the readme for the tools/mccibooatloader_image regarding the clang compile issue faced.

Please check and merge.

[terrillmoore]

Thanks for the contribution. The underlying issue is real -- clang on Windows needs the UCRT headers from the Windows SDK -- but this PR needs some rework before it can be merged.

1. Terminology: "Visual Studio SDK" refers to the VS extensibility SDK, not the Windows SDK. The prerequisite here is the Windows SDK (specifically the UCRT headers that ship with it). The link points to the right download page but the name is misleading.

2. Workaround is too aggressive: Telling users to uninstall other SDK versions is likely to break other tooling. A better approach would be to document how to point clang at the correct SDK (e.g., setting environment variables, or using --sysroot/-isystem flags), or to fix the Makefile to handle this.

3. Writing style: The surrounding README is terse and direct. The added text is verbose by comparison ("It's important to check whether the work/test machine has installed with any of the Visual Studio SDK versions..."). Please tighten it up to match the existing tone.

4. Suggested direction: A concise note like "You'll need the Windows SDK installed for UCRT headers. If clang fails to find headers and you have multiple SDK versions under C:\Program Files (x86)\Windows Kits\10\Include, set the INCLUDE environment variable to point at the correct version." would be more useful and less risky than the current text.

Please update based on this review.