README: remove 'Issue #XXX' placeholder and clarify subplot warning

[krystophny]

Category: docs
Priority: P3

Problem

• README contains a placeholder reference ("Issue #XXX") in the Subplot Grids note.
• User-facing docs should not ship with placeholders, and the warning text can be tightened.

Evidence

• README.md: lines 34-36 currently show the placeholder and warning text.

Solution

• Remove the placeholder reference.
• Clarify that multiple subplots are not yet implemented and that calling subplot() emits a warning and only the first panel is rendered/saved.