[docs] add a lessons learned document

LESSONS_LEARNED.md

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+# Wrong decisions
+- **tauri-leptos** adds unnecessary complexity to the project.
+  Tauri is built to transfer backend Rust stuff to the frontend JavaScript world.
+  But with leptos, the frontend becomes Rust so you have to transfer everything again back into the Rust world.
+
+# Good decisions
+- **Tauri** is strong 💪
+  - Easy project setup with `cargo install create-tauri-app --locked` and `cargo create-tauri-app`
+  - Easy testing with `cargo tauri dev`
+  - Easy bundling (with installers and everything) with `cargo tauri build`
+  - Final binary size of my tauri-leptos project is 13MB (which seems small to me if you think that a whole WASM / Web stack is in there)
+
+# Open questions
+- **logging**: I don't understand the relationship (or lack thereof) between `tracing` and `log`.
+  I had to refactor logging in the client away from `tracing`.