Add FFmpeg tips.

Based on
https://unix.stackexchange.com/questions/28803/how-can-i-reduce-a-videos-size-with-ffmpeg

void/reference/ffmpeg.md

@@ -0,0 +1,14 @@
+# FFmpeg
+
+## Resize video
+
+Base command: `ffmpeg -i <input> <output>`
+
+Intermediary options:
+
+- `-vcodec libx265|libx264`
+- `-crf <value>` reasonable range for h265 is 24 to 30, 34 still looks like something
+- `-b <number>k` bitrate (per second)
+- `-fs <number>` file size limit
+- `-vf "scale=trunc(iw/4)*2:trunc(ih/4)*2"` filter to scale by half
+- `-vf scale=1280:720` filter to scale to 720p