docs: complete documentation for Milestone 4

- Rewrite README.md with overview, requirements, setup, run, tests
  sections
- Add docs/flowchart.md with Mermaid diagram and link to diagrams.net
  source
- Update docs/plan.md to mark Milestone 4 as complete

docs/flowchart.md

@@ -0,0 +1,17 @@
+# Blind Auction Flowchart
+
+Original chart: [diagrams.net](https://viewer.diagrams.net/?highlight=0000ff&edit=_blank&layers=1&nav=1&title=Blind%20Auction%20Flow%20Chart)
+
+```mermaid
+flowchart TD
+    A([Start]) --> B[Print welcome message]
+    B --> C[bids = empty dictionary]
+    C --> D[/Ask name/]
+    D --> E[/Ask bid, repeat until valid/]
+    E --> F[Store bids name = bid]
+    F --> G{Other bidders?}
+    G -- yes --> H[Clear screen] --> D
+    G -- no --> I[Find highest bid]
+    I --> J[/Print winner/]
+    J --> K([End])
+```