examples: add README file with simple compilation instructions

Makefile

@@ -41,7 +41,7 @@ FFLIBS-$(CONFIG_SWSCALE)  += swscale
 FFLIBS := avutil
 
 DATA_FILES := $(wildcard $(SRC_PATH)/presets/*.ffpreset) $(SRC_PATH)/doc/ffprobe.xsd
-EXAMPLES_FILES := $(wildcard $(SRC_PATH)/doc/examples/*.c) $(SRC_PATH)/doc/examples/Makefile
+EXAMPLES_FILES := $(wildcard $(SRC_PATH)/doc/examples/*.c) $(SRC_PATH)/doc/examples/Makefile $(SRC_PATH)/doc/examples/README
 
 SKIPHEADERS = cmdutils_common_opts.h
 

doc/examples/README

+FFmpeg examples README
+----------------------
+
+Both following use cases rely on pkg-config and make, thus make sure
+that you have them installed and working on your system.
+
+
+1) Build the installed examples in a generic read/write user directory
+
+Copy to a read/write user directory and just use "make", it will link
+to the libraries on your system, assuming the PKG_CONFIG_PATH is
+correctly configured.
+
+2) Build the examples in-tree
+
+Assuming you are in the source FFmpeg checkout directory, you need to build
+FFmpeg (no need to make install in any prefix). Then you can go into the
+doc/examples and run a command such as PKG_CONFIG_PATH=pc-uninstalled make.