engine: Shift on the okurigana asks for that reading, as SKK does

kaku offers the twenty-two readings of かく before 書く, because the SKK
dictionary keys a verb by its stem and gives no frequency to rank the two
lists by.  SKK's own answer is the shift key: kaKu says where the
okurigana starts, and that split now comes first — the reading's own
candidates still follow it.  Caps Lock sends no Shift, so it marks
nothing, and a word typed without Shift is unchanged.

README.md

@@ -39,6 +39,10 @@ current page; before that the digits type. `Enter` commits the chosen candidate,
 reading when none is chosen, as `0` always does; so does typing on, or any
 key that ends the composition. `Backspace` deletes the last kana shown;
 romaji that made no kana stays in the reading as typed until it is mended.
+The dictionary keys a verb or adjective by its stem, so `kaku` offers the
+readings of かく and then 書く and its kin; typing the okurigana with
+`Shift`, as SKK does — `kaKu` — asks for that split first.  Caps Lock is
+not `Shift` and marks nothing.
 `Backspace` and `Esc` on a chosen candidate go back to the reading, and
 `Esc` on the reading cancels it. Katakana mode does not perform Kanji
 conversion.