Add user manual

doc/manual.md

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+# Genoa user manual
+
+Genoa is a tool to drawings out of simple scripts and export them as SVG, or preview them inline for rapid turnaround. Genoa shapes and options only cover a subset of SVG. Note that the preview isn't always exact, especially at small sizes and/or when it contains text.
+
+Genoa scripts are based on a safe subset of the Tcl programming language. A Tcl tutorial is outside the scope of this manual; see the examples to get started.
+
+That said, certain procedures are specific to Genoa.
+
+## Cheatsheet
+
+Shapes:
+
+```tcl
+line $x1 $y1 $x2 $y2
+box $x $y $width $height
+circle $cx $cy $r
+oval $cx $cy $rx $ry
+text $x $y
+```
+
+Options:
+
+```tcl
+# Can be a named color, hex triplet or rgb(n, n, n) code.
+# Use an empty string for transparency.
+color $c
+fill $f
+# Width of lines / borders (default: 1).
+width $w
+# Font options
+font $f
+size $s
+```
+
+Other:
+
+```tcl
+# Defines what part of the drawing will be visible.
+# Can be placed anywhere in the script.
+viewport $x $y $width $height
+repeat $times {
+	# Just what it looks like
+}
+# Useful for debugging
+alert $message
+```
+
+This cheatsheet applies to Genoa version 0.3.2 alpha from September 2023.