Doc: add flowchart (#1328)

The flowchart will be kept updated:

- Use [drawio](https://app.diagrams.net/) to edit.

1. Open [drawio](https://app.diagrams.net/) in brower
2. Menu → File → Open from ... → Device:

Select [FlowChart.html](FlowChart.html)

3. Edit flow chart
4. Export to svg:

Menu → File → Export as... → SVG...

docs/Developer.md

@@ -0,0 +1,17 @@
+
+## Flow chart
+
+![FlowChart](drawio/FlowChart.svg)
+
+
+- Use [drawio](https://app.diagrams.net/) to edit.
+
+1. Open [drawio](https://app.diagrams.net/) in brower
+2. Menu → File → Open from ... → Device:
+
+Select [FlowChart.html](FlowChart.html)
+
+3. Edit flow chart
+4. Export to svg:
+
+Menu → File → Export as... → SVG...