update readme, mention wiki

README.md

@@ -95,10 +95,14 @@ rga should compile with stable Rust (v1.36.0+, check with `rustc --version`). To
 
 ## Available Adapters
 
+rga works with _adapters_ that adapt various file formats. It comes with a few adapters integrated:
+
 ```
 rga --rga-list-adapters
 ```
 
+You can also add **custom adapters**. See [the wiki](https://github.com/phiresky/ripgrep-all/wiki) for more information.
+
 <!-- this part generated by update-readme.sh -->
 
 Adapters:
@@ -121,7 +125,7 @@ Adapters:
 
 - **ffmpeg**
   Uses ffmpeg to extract video metadata/chapters, subtitles, lyrics, and other metadata  
-   Extensions: .mkv, .mp4, .avi, .mp3, .ogg, .flac
+   Extensions: .mkv, .mp4, .avi, .mp3, .ogg, .flac, .webm
 
 - **zip**
   Reads a zip file as a stream and recurses down into its contents