Add documentation (docs/index.md)

docs/index.md

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+# Corrscope Help
+
+*Corrscope is named because it cross-correlates the input wave and a history buffer, to maximize wave alignment between frames.*
+
+## Program Layout
+
+![Screenshot of Corrscope and video preview](images/corrscope-screenshot.png?raw=true)
+
+- Global options: Left pane
+- Trigger options: Top-right pane
+- Per-channel options: Bottom-right table
+
+## Correlation Trigger Explanation
+
+Unlike other triggering algorithms (which only depend on the current frame's worth of data), Corrscope uses a correlation trigger, which remembers several forms of data between frames, to help align future frames.
+
+### Options
+
+- Left pane, Global
+    - `Trigger Width` (also controllable via per-channel "Trigger Width ×")
+- Top pane, Wave Alignment
+    <!-- - `Buffer Strength` -->
+    - `Buffer Responsiveness`
+    - `Mean Responsiveness`
+    - `Pitch Tracking`
+- Top pane, Edge Search
+    - `Edge Direction`
+    - `Edge Strength`
+
+### Variables Remembered
+
+- `buffer`: array of samples, containing `Trigger Width` (around 40 milliseconds) recent "trigger outputs". Starts out as all zeros.
+- `mean`: real number, estimated from recent "trigger inputs". Starts out at 0.
+
+### Obtaining Data (each frame)
+
+On each frame, corrscope fetches [from the channel] a buffer of mono `data`, centered at the current time. The amount of data used is controlled by `Trigger Width`, which should be increased to keep low bass stable.
+
+To remove DC offset from the wave, corrscope calculates the `new mean` of input `data`. Because each frame's mean may jitter depending on what window of the wave was selected, update saved `mean` = `mean` + `Mean Responsiveness` × (`new mean` - `mean`). Then corrscope subtracts this averaged `mean` from `data`.
+
+Corrscope then estimates the fundamental `period` of the waveform, using autocorrelation.
+
+### (optional) Pitch Tracking
+
+If `Pitch Tracking` is enabled:
+
+If `period` changes significantly, corrscope computes the spectrums of `data` and `data` from 2 frames ago, and cross-correlates them to estimate the pitch change over the last 2 frames. It then resamples (horizontally scales) `buffer` to match this pitch change.
+
+### Correlation Triggering (uses `buffer`)
+
+Corrscope multiplies `data` by `data window` of some sort.
+
+Corrscope uses `edge_finder`, which is computed once and reused for every frame.
+
+Corrscope correlates `data` with `buffer` + `edge_finder` to find the best part of `data` to pick, then sets `position` to be used for rendering.
+
+### (Optional) Post Triggering
+
+If post triggering is enabled, the post trigger is called with `position` and returns a new `position`, which overwrites the original variable.
+
+### Updating Buffer
+
+- Corrscope fetches [from the channel] a buffer of mono `triggered data`, centered at `position`
+- `triggered data` is multiplied by a Gaussian window with width 0.5 × period
+- Corrscope updates `buffer` = `buffer` + `Buffer Responsiveness` × (`triggered data` - `buffer`).