INSTALL: Add note about libav* configuration

Author: PromyLOPh

INSTALL

@@ -8,11 +8,15 @@ Dependencies
 - pthreads
 - libao
 - gnutls
-- gcrypt (with blowfish cipher enabled)
+- gcrypt[1]
 - json-c
-- libav/ffmpeg
+- libav/ffmpeg[2]
 - UTF-8 console/locale
 
+[1] with blowfish cipher enabled
+[2] required: demuxer mov, decoder aac, protocol http and filters volume,
+    aformat, aresample
+
 Building
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