added TODO

TODO.md

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+# TODO
+
+- [x] Basic participants
+- [x] Basic sequences
+- [x] Separators
+- [x] Gaps
+- [x] Groups
+- [x] Self arrows
+- [x] Arrow from start / to end, small arrows
+- [x] Lifelines
+- [x] Different types of participants
+- [x] Notes
+- [ ] Synchronized arrows
+- [ ] Slanted arrows
+- [ ] Different types of arrow tips (WIP)
+- [ ] Fix column spacing with notes over multiple columns
+- [ ] Fix notes with arrows from start / to end / small arrows
+- [ ] Fix group size with self arrows + notes
+- [ ] Unify styling structure
+- [ ] Add args verification to catch user errors + pretty error messages
+- [ ] PlantUML parser
+- [ ] (Message numbering)
+- [ ] Different types of groups (alt/loop/etc.)
+- [ ] Delays
+- [ ] Auto-fit in parent