readme: add dpdk shared library note to LD_LIBRARY_PATH

Change-Id: Ia9d525bb7d0233eaa76f4c59e8f956e87123afd8
Signed-off-by: Changpeng Liu <changpeng.liu@intel.com>
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/4376
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>

README.md

@@ -190,13 +190,16 @@ to do the following steps:
 - run ldconfig specifying the directory containing SPDK shared libraries
 - provide proper `LD_LIBRARY_PATH`
 
+If DPDK shared libraries are used, you may also need to add DPDK shared
+libraries to `LD_LIBRARY_PATH`
+
 Linux:
 
 ~~~{.sh}
 ./configure --with-shared
 make
 ldconfig -v -n ./build/lib
-LD_LIBRARY_PATH=./build/lib/ ./build/bin/spdk_tgt
+LD_LIBRARY_PATH=./build/lib/:./dpdk/build/lib/ ./build/bin/spdk_tgt
 ~~~
 
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