docs: add routine provenance (video + source thread) to README

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

README.md

@@ -9,6 +9,31 @@ Built with **Svelte 5 + Vite** as a fully static SPA. Music theory is powered by
 [`tonal`](https://github.com/tonaljs/tonal). Timing uses a Web Audio lookahead
 scheduler so the metronome never drifts.
 
+## Where this routine comes from
+
+The drill set began with the YouTube video
+[**“How to get SO GOOD at guitar it feels ILLEGAL (Using Neuroscience)”**](https://www.youtube.com/watch?v=7SNcy_GprQU).
+Its practice principles were turned into a concrete, coded routine — the R/F/T/L/E
+drills kept in my Obsidian wiki — in [this conversation](https://claude.ai/share/dfded9f7-5a26-4ea7-b265-fab118e32ed7).
+This app then gives each of those drill codes an interactive helper so the routine
+is actually runnable next to the guitar.
+
+The routine is built on a handful of evidence-based practice ideas from the video:
+
+- **Interleaving + active recall** — mix skills within a session and test yourself
+  cold, instead of mindless blocked repetition of the same lick.
+- **A practice journal** as the engine that makes the rest work.
+- **Short, frequent sessions** over long grinds.
+- **The additive approach** — build a passage up in tiny increments.
+- **Deep practice / myelin** — effortful reps right at the edge of your ability.
+- **Mental practice** away from the instrument.
+
+Several of these are baked into the app: the “clean? +2” tempo bump enforces
+additive, edge-of-ability progression (only speed up once a rep is clean);
+per-drill BPM persistence supports short daily sessions and a journal-like record
+of where you left off; and the mixed drill categories on the index page nudge you
+toward interleaving rather than grinding one skill.
+
 ## Develop
 
 ```bash