README.md: add info about audio and video indexing

Author: Asd-g

README.md

@@ -10,6 +10,24 @@
 
 If you have issues about the file names save the AviSynth script (`.avs`) as UTF-8 or just enable UTF-8 code pages in Windows (if available) - `Windows Settings > Time & language > Language & region > Administrative language settings > Change system locale, and check Beta: Use Unicode UTF-8 for worldwide language support.`
 
+##### Note about decoding audio and video from the same file in the same script
+
+Always prefer this order
+
+```
+audio = LWLibavAudioSource()
+video = LWLibavVideoSource()
+```
+
+over this order
+
+```
+video = LWLibavVideoSource()
+audio = LWLibavAudioSource()
+```
+
+In the second case the file will be scanned twice and the video info will be written twice (overwritted) in the index file.
+
 ##### FFmpeg
 
 [This](https://github.com/HomeOfAviSynthPlusEvolution/FFmpeg/tree/custom-patches-for-lsmashsource) is the recommended FFmpeg version.