readme: add missing documentation

Co-authored-by: Oneric <oneric@oneric.stub>

README.md

@@ -118,6 +118,19 @@ When creating an instance of SubtitleOctopus, you can set the following options:
   occurs if browser doesn't support web workers). (Optional)
 - `debug`: Whether performance info is printed in the console. (Default:
   `false`)
+- `renderMode`: Rendering mode. (If not set, the rendering mode is determined by the `blendRender` and `lossyRender` options)
+  - `normal` - Default
+  - `blend` - WASM Blending
+  - `fast` - Fast Render Mode (Lossy) (EXPERIMENTAL)
+- `dropAllAnimations`: Remove all animation tags, such as karaoke, move, fade, etc. (Default: `false`)
+- `libassMemoryLimit`: libass bitmap cache memory limit in MiB (approximate) (Default: `0` - no limit)
+- `libassGlyphLimit`: libass glyph cache memory limit in MiB (approximate) (Default: `0` - no limit)
+- `targetFps`: Target FPS (Default: `30`)
+- `prescaleTradeoff`: Scale down (`< 1.0`) the subtitles canvas to `softHeightLimit` to improve speed, or scale it up (`> 1.0`) to improve quality. (Default: `null` - no scaling)
+- `softHeightLimit`: The height to which the subtitles canvas will be scaled. (Default: `1080`)
+- `hardHeightLimit`: The maximum height of the subtitles canvas. (Default: `0` - no limit)
+- `renderAhead`: How many MiB (approximate) of subtitles to render ahead and store. (Default: `0` - don't render ahead)
+- `resizeVariation`: The resize threshold at which the cache of pre-rendered events is cleared. (Default: `0.2`)
 
 Additionally there are options to choose between different rendering modes, which are detailed
 below. If multiple rendering options are set any of them may be used, they are not additive.
@@ -142,6 +155,24 @@ The Fast Render mode has been created by @no1d as a suggestion for fix browser f
 
 **WARNING: Experimental, not stable and not working in Safari**
 
+#### Render Ahead (WASM Blending with pre-rendering) (EXPERIMENTAL)
+Upon creating the SubtitleOctopus instance, set `renderAhead` in the options to a positive value to use this mode.
+In this mode, SubtitleOctopus renders events in advance (using WASM blending) so that they are ready in time.
+The amount of pre-rendered events is controlled by the `renderAhead` option.
+Each pre-rendered event is provided with information about its start time, end time, and end time of the gap after (if any).
+This mode will analyse the events to avoid rendering empty sections or rerendering non-animated events.
+Resizing the video player clears the cache of pre-rendered events (the threshold is set by `resizeVariation`).
+
+> The `renderMode`, `blendRender` and `lossyRender` options are ignored.
+
+> **WARNING: Experimental, may stall on heavily animated subtitles**
+This mode tries to render every transition - at worst, every frame - in advance.
+If the rendering of many frames takes too long and the cache of prepared frames gets depleted
+(e.g. during a long section with heavy animations), the current subtitle-frame will continue to
+be displayed until the prerendering can catch up again.
+Adjusting `prescaleTradeoff`, `softHeightLimit` and `hardHeightLimit` to lower the resolution of
+the rendering canvas can work around this at the expense of visual quality.
+
 
 ### Brotli Compressed Subtitles
 The SubtitleOctopus allow the use of compressed subtitles in brotli format, saving bandwidth and reducing library startup time