update readme

README.md

@@ -1,18 +1,40 @@
 # ugoira-downloader
 Downloader for Pixiv's [Ugoira](https://safebooru.donmai.us/wiki_pages/ugoira?z=1) animation format.
 
+## Dependencies
+### Internal
+Only Go's standard library.
+
+### External
+- FFmpeg: for Ugoira to video conversion.
+
 ## Usage
-```
-ugoira-downloader post_id
+### Downloading a Ugoira
+```sh
+ugoira-tool download <post_id>
+# with specific file name:
+ugoira-tool download -o ugo.zip <post_id>
 ```
 By default, it will download both the zip file (containing the frames) and the manifest
 JSON in the current working directory (default filenames being `<post_id>_ugoira.zip` and `<post_id>_ugoira.json` respectively).
 Use `-o` to change the zip file path and `-M` for the manifest.
 
-See `ugoira-downloader -h` for all flags.
+### Conversion of Ugoira animation to WebM
+```sh
+ugoira-tool u2v -o anim.webm <ugoira_file> <ugoira_manifest>
+# Example:
+ugoira-tool u2v -o anim.webm ugo.zip ugo.json
+```
+The created WebM file uses the VP9 codec with flags that should yield a decent quality while
+maintaining a reasonable file size, avoiding
+any additional quality degradation (since most Ugoira use JPEG for their frames).
+
+The default codec flags are in `ffmpegDefaultArgs` at `conversion.go`.
+
+Additionally, you can set your own codec flags with `-ffmpeg-args`
 
 ## Building
-```
+```sh
 go build .
 ```