doc: clarify configure features

This way people will not get surprises regarding the current configure
behaviour and contributors will have a guideline on what is expected.

INSTALL

@@ -9,3 +9,10 @@ path when launching 'configure', e.g. '/libavdir/libav/configure'.
 2) Then type 'make' to build Libav. GNU Make 3.81 or later is required.
 
 3) Type 'make install' to install all binaries and libraries you built.
+
+NOTICE
+
+ - Non system dependencies (e.g. libx264, libvpx) are disabled by default.
+
+ - The default cflags include -g, if you want lean libraries you can either
+   pass --disable-debug or strip the debug symbols at a later time.