docs: clarify URL field behavior in JSON manifest format

[EIrwin]

Scope of Change

This PR adds important documentation clarification to the README.md file regarding JSON manifest format behavior. Specifically, it adds a warning section that explains how the url field interacts with other URL-related fields.

What the change does:

• Adds a new warning section "⚠️ Important: URL Field Behavior" to the README.md
• Clarifies that when url field is present, all other URL-related fields are ignored
• Provides clear examples of what works vs. what doesn't work
• Explains the two supported manifest format approaches

Parts of code modified:

• README.md only (documentation change, no code modifications)

Background

Users were experiencing confusion when trying to use both url and filesystem fields (littlefs, spiffs, fatfs) in the same manifest, expecting both to work. The current library behavior (as implemented in checkJSONManifest()) ignores all other URL-related fields when url is present, but this wasn't documented.

Known Limitations

• This is a documentation-only change
• No functional code changes are included
• The underlying library behavior remains unchanged (by design)

Testing

• Verified the markdown formatting renders correctly
• Confirmed the examples accurately reflect the current library behavior as implemented in the source code
• No functional testing required as this is documentation only

This change helps prevent user confusion and makes the manifest format requirements explicit, improving the developer experience when working with filesystem updates.

[chrisjoyce911]

Thanks for your work