docs: describe saved colors (Issue #56)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -23,7 +23,7 @@ The interface is available in English and German; ink-notes follows the system l
 
 ### Writing and drawing
 - **Seven pens** in the toolbar, each with its own color and width (0.15–5 mm, pressure-sensitive). When you change a pen's color or width, it remembers that – also across restarts. **Highlighter** (semi-transparent, 4–8 mm) in four colors; it does not get darker within one stroke, only a new stroke covers more strongly.
-- **Custom colors:** Every color in the toolbar (pens, highlighter, text) can be changed – tap the selected color again and pick a preset or any shade with 🎨. The toolbar remembers the colors. They look the same on screen as in PDF and image exports.
+- **Custom colors:** Every color in the toolbar (pens, highlighter, text) can be changed – tap the selected color again and pick a preset or any shade with 🎨. The toolbar remembers the colors. Under the presets, “➕ Save color” keeps the current shade in “My colors” (up to 12, shared by pens, highlighter and text; 🗑 removes the selected one). They look the same on screen as in PDF and image exports.
 - **Two-stage smoothing:** While you write, a 1€ filter removes jitter and the line appears immediately. When you lift the pen, the stroke is refined: corners (bends over 65°) are kept and drawn round, the pressure curve is smoothed, redundant points are dropped and the ends become round.
 - **Pressure curve** adjustable to your pen in the ☰ menu (softer/harder, threshold against base noise).
 - **Pen buttons** can be assigned in the ☰ menu: erase (default), select, highlighter, pen or nothing – active while the button is held. Each button appears there after it is pressed for the first time.