documenting grooves

doc/8-advanced/grooves.md

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+# grooves
+
+Grooves are macros for speed.
+
+A groove is the equivalent of repeating `0Fxx` commands on each row to get a cycle of speeds. For example, a groove of "6 4 5 3" makes the first row 6 ticks long, the next row 4 ticks, then 5, 3, 6, 4, 5, 3...
+
+
+![groove](groove.png)
+
+To set the song's groove:
+- Open the "Speed" window.
+- Click the "Speed" button so it becomes "Speeds" (effectively a groove of two speeds).
+- Click again so it becomes "Groove".
+- Enter a sequence of up to 16 speeds.
+
+
+![groove patterns](grooves.png)
+
+The "Grooves" window is for entering preset groove patterns.
+- The `+` button adds a new groove pattern; click in the pattern to edit it.
+- The `×` buttons remove them.
+
+A single `09xx` command will switch to the matching numbered groove pattern.